To help OP understand, basically it is a fish generator and produces infinite fish over time. You must place the structure next to an active fish entity though otherwise it won't work. If you play on island worlds this isn't really an issue, but if you play on a world with very limited water this may be useful in some form long as you don't fish everything before you place one.
So, long explanation short, it generates fish if placed next to fish entites.
A mod that would go well with this is 'Catch and Release' which allows you to catch placeable entities of the fish, so if you play with waterfill mods that let you make ponds or you fish out your water sources completely, you can catch a fish and relocate it to your pond and then use this mod to generate more fish for said pond.
