A composter is a block that converts some biological material into bone meal. It also serves as a farmer villager's job site block.

Do composters in Minecraft help crops grow?

Area of Increased Growth- Same as a Water Block in a plowed field. So basically when you make a farm place a Compost Bin on top of the Water Block and fill it up and all crops grown in the area will grow twice as fast as normal.

What do villagers do with composters?

Farmer villagers will fill up the composter with any excess seeds they have in their inventory. Once the composter is full, the villager will take the bone-meal and use it on the crops.