During a game, the catcher and the pitcher have to both know what pitch is being thrown so the catcher can catch it - it’s hard to catch breaking balls unless you know to expect one. So the game has evolved that the catcher normally “calls” games, eg decides what pitch the pitcher is going to throw. Catchers make careers on how well they call games. They are the smartest players on the field, and have to remember the stats and approaches for every batter on the other team, not only for the starting pitcher that day but also all the relievers that might be used in a game. They communicate the pitch to be thrown to the pitcher using hand signals, normally with the non-catching hand on the inside of their thigh so the batter can’t see the sign.

But anyone behind the pitcher can of course see the sign, and after seeing a few tens of pitches at most can figure out which sign corresponds to which pitch. Knowing what pitch is coming is a significant advantage to the hitter.

It’s legal for a runner on second to watch the catcher and convey those signs to the pitcher, but in MLB it’s strictly illegal to use “technology” - meaning, mostly, optics or electronics.

The catcher needs to know what sort of pitch the pitcher will throw so he can set up correctly to catch the pitch. To make sure catcher and pitcher are on the same page, each pitch starts with the catcher giving hand signals to the pitcher to suggest he throws a specific type of pitch. These hand signals are what is meant by "signs."

An opposing team can "steal signs" by looking at the catcher while he's giving his signs and then somehow relay that information to the batter. That way the batter knows what type of pitch is coming. Usually the information is given to the batter by some other type of signal, that's what "banging on the garbage can" was, a signal that a specific type of pitch was coming. (In this case they were signaling if a pitch was going to be an offspeed pitch)

So basically what happened was Astros set up a camera in the outfield aimed at the opposing team's catcher. The catcher would signal for, for example, a breaking ball. Astros employee viewing video feed would see this and then bang on the garbage can to indicate to the batter that the pitch would be offspeed.

It's important to know that "stealing signs" the old fashioned way (without cameras/technology) is legal and part of the game. This is why catchers will give multiple signs when runners are on base, because base runners may be able to look at the catcher while he's giving his signs and relay information to the batter. When the catcher is using multiple signs there's an agreement beforehand between the catcher and the pitcher which of the multiple signs is the "real" sign.

There's a Wikipedia page on sign stealing: https://en.wikipedia.org/wiki/Sign_stealing

Hopefully someone who actually knows baseball can correct me if I get this wrong, but:

In baseball, the pitcher stands in the middle of a diamond-shaped field. They throw the ball towards the opposing team's batter, who tries to hit it. The pitcher's goal is to get the ball past the opposing batter to the catcher, who is squatting on the ground behind the batter.

The catcher, having full view of all the players and being close to the batter, can opt to signal to the pitcher how best to throw the ball to get it past the batter. For example, they may raise 1 finger to throw it as fast as possible ("fastball"), 2 to make it _seem_ look a fastball but actually slow it down ("changeup"), 3 to curve the ball, etc. The catcher does this such that only the pitcher has a good view of it. This is called a "sign".

It's legal for the opposing team to try to see what the catcher is signaling and inform the batter, but not to do so with electronic equipment. The Astros were using video cameras to get a close look at the catcher's signs, then banging a metal drum to inform the batter of what kind of pitch was being signaled.

The article has an embedded Tweet which has a good video demonstrating all this.

I'm basing this on a single article I read that was a little more detailed.

A "sign" is a hand signal the catcher makes to the pitcher, indicating which pitch (fastball / curveball / whatever) the pitcher will throw. If the pitcher wants to throw something else, he shakes his head, and the catcher signals something different. Repeat until the pitcher is happy.

The pitcher's rejection is visible to everyone, but the catcher's signal is hidden.

"Stealing signs" does not refer to stealing the key that translates between hand signals and pitch types, so that you can tell by looking at the signal what pitch will be thrown. That's considered obvious. (I mention this because I assumed that's what "stealing signs" would mean.)

Instead, "stealing signs" refers to looking at the catcher when he signals, so that you see the signal. Interpreting it is then up to you. Oddly, this is explicitly legal if you do it by having a runner on second base, but illegal by any other means.

The electronics involved are a small camera mounted somewhere aimed at the catcher. The banging noise is a simple code -- one bang for signal A; two bangs for signal B; three bangs for signal C... the function is to communicate the signal from the guy off in the bowels of the stadium viewing the camera feed, to the batter who wants to know what pitch to prepare for.

The catcher will signal a pitch with their hand between their legs. One finger for a fastball, two for a curve, something like that. The reason this communication is necessary is so the catcher sets up correctly to catch e.g. a breaking ball.

What the Astros did was zoom in a camera from the outfield to see this and feed that video straight into the dugout. The person in the dugout with the feed would bang a garbage can (or something) to signal audibly to the person up at bat. Knowing what pitch is coming is a massive advantage for a batter.

It really has to happen in a matter of a second or two in order to work.

Before the pitcher throws the ball, the catcher signals with his fingers what kind of pitch to throw.

Pitchers switch between straight fastballs and slower curveballs and a pitch called a change up that looks like a fastball delivery but comes across the plate slower.

Obviously if the hitter knew what to expect he’d do a lot better. So the astros had a camera that was looking at the signs and feeding them to the dugout. Then when they knew that a good pitch to hit was coming, they’d make a banging sound. It’s not actually super sophisticated.

You’re actually allowed to steal signs from like a runner on second — you’re not allowed to use a camera to do it.

A sign is when the crouching catcher flashes a hand gesture between their legs to indicate to the pitcher which type of pitch to throw against the batter. The Astros adjusted the placement of a camera to better see the catcher’s signs, someone on a laptop in the locker room watching the camera would see the sign, bang on a trash can to make a loud noise the dugout and presumably the batter to know when (usually) a fastball was coming. Fastballs have less curve on them, making it easier for the batter to hit when they know one’s coming.