It refers to the way the circuit board is designed. Older keyboards were generally designed with the intention that only one, maybe two, keys would be held down at once.

But, modern applications (especially games), often require or support many keys to be pressed at once. This has required keyboard circuits to be designed to handle that. Ghosting is what happens when it's not designed to handle that (the computer registers key presses that weren't actually pressed) and N_Key Rollover is what you get when you eliminate ghosting and allow N number of keys to be pressed at once.

Basically, in a keyboard several keyswitches share the same wires. Thus when pressing keys on the same wire it can happen that the controller recognize the wrong key or multiple keys. This can be solved and depending on how many keys you can press at the same time it's called 2-key,3-key, etc. or n-key when there is no limit.

Should be noted that USB is limited by design to 6-key, only PS2 allows AFAIK true n-key.

You can look up matrix-wiring and diodes in keyboards if you want a more realistic explenation regarding the problems in keyboard-design

Find some program that records key events, and hold down q, w, and a. It's somewhat likely that d will trigger as well (or that only two keys will trigger). That's ghosting (or jamming) due to only 2-key rollover.

It happens because there's not an individual electrical connection for each key. Instead, you have one wire for each row and column, and keys connect between them.

Guaranteed not to be haunted.