what are .lua files used for

looking to find what a .lua does in computing as im new to it

Lua is a scripting language. It's used by many games for modifications

I have a gmod darkrp server and lua files are how I add addons and things like that.

https://www.lua.org/ the official lua site. It can be used for more then just addons for gmod lol

its a scripting language, usually used for light plugins used in servers etc.
some games that use it are:
civ 5, gmod, far cry, wow, etc