So just what are earthworm castings?

I just bought a bag of Fox farm Ocean Forest due to numerous recommendations. The bag goes on about earthworm castings. What are these, what do they do for my soil and plants, and do they require any additional maintenance? Or is it nothing I need to worry about.

Also, I bought a few bags of Perlite. What is a good perlite/soil ratio?

Provides many nutrients for the plants...good ratio is....60% soil 15%perlite 10%sand 10% premix and 5% worm castings.

its worm shit, they eat dirt and then crap out dirt but it has beneficial microorganisms and other stuff after its processed by the worm.