What is debug used for?

I randomly typed in 'debug' a while back, before my studio broke, but I'm not sure what it does? I meant to ask this a while back, but I forgot to.

debug is one of Lua's libraries. Roblox, as with most all default lua libraries, such as io, has removed the debug library.

Other than that, it's just a library with useful methods/functions for debugging.

Trying to use it in roblox is useless, as it defaults to nil and will act like an unset value, unless you set it to something.