Moss farming is using the feature of bone-mealing a moss block and getting more moss blocks, as well as azalea and flowering azalea (which is a more efficient way to farm oak wood), moss carpet, seeds and grass. You can also farm bonemeal using moss, because it produces more bonemeal than it uses.
You can also use the moss block's mechanics to terraform an area since it replaces most blocks around it with moss blocks. Moss blocks are hard to obtain without bonemealing existing moss blocks, with the only other methods being to find them from shipwrecks or buy them from wandering traders, so having a way to farm them can be very useful.
Automatic moss farming
Moss blocks can be farmed automatically by bonemealing stone made by a stone generator, since cobblestone is not able to be replaced by moss.
They can be harvested with pistons since they will be crushed if they do, or they can be harvested manually with a hoe. You can then transport the drops using water flows which can carry it to a collection system, such as a hopper minecart one, which, if you want, you can compost and then turn into more bonemeal.
This design by Shulkercraft is a self-sustaining moss bonemeal farm.