In Minecraft, zombies drop feathers, and feathers are used for making arrows. Normally, you have to go out and kill zombies or chickens manually to get them. What this guy did was make a trap where zombies spawn and fall into flowing water. The water flows to a field of cacti, where the zombies get stuck and die (cacti hurt you to touch them). Some of the zombies drop feathers, which continue flowing with the water to an opening. In effect, the guy gets feathers delivered to him automatically, with no effort.

You can rig up similar contraptions for other resources, like a "bacon farm" which delivers pork (pork heals you).