A compost biome is what it says, a whole biome where the ground is made of compost.
For starters to find this thing it will be a 1 in 1,250 chance (0.08%) of spawning in your world. When you find it you will have a stash of compost waiting to be harvested. It will be mixed in with dirt so at minimum you may find 129 compost, but at maximum you may find upwards of 2,359
And just maybe a 1 in 10,000 (0.01%) chance of finding a special tree that grows compost.
That’s literally about it. May not be anything special. But will provide a decent farm if you find one…