Basically allows you to craft slime from moss blocks, since moss are able to make slime (So Steve basically crush a moss block and squeeze the slime out of the block)
Moss blocks in a 3 x 3 crafting grid will result into 3 slime or
if you have an empty bottle, right click a frog and you'll get a bottle of slime
2 bottles of slime = 1 slimeball
Frogs needs 5-10 mins to be able and get a new bottle of slime