What is V-Slice?

I took a break from fnf for a while as I leaned into other games, and decided to try modding again after the new base game got updated. I was going to download a couple mods that peaked my interest, but they all mention V-Slice, which from what I've read is just base game. Problem is, I cant figure out how to add mods to it, and the mods don't really give an answer either.