A compost tumbler is the stomach of the compost digestion system. It aerates and mixes the compost so the the bacteria reach the new stuff quickly. This one is a barrel that pivots on a horizontal axle. Every time you throw in new compost, turn the thing over.
I built one because the rich food waste that was going into the Pentagon composter
was causing smell and fly problems. Hopefully the tumbler will solve that.
There are several instructables showing good ways to make a tumbler. There isn't a bad way to make one. I threw this one together in about an hour.