What does require do? I see a script with only "require(259230115)" inside it and it's in serverscriptservice with http enabled. It's able to make guis and such appear but the only coding that I found is that one line.
The keyword require
is used to load module scripts so that they can be used inside of the given script.
The function require takes one variant argument, either a reference to a module script or an integer. The integer variant is used to load a module script by it's asset id, assuming it has been uploaded to the Roblox website.
See this wiki page for more information and usage examples.
Require makes it so the rest of the script won't run unless that object exists, so lets say that I do require(game.ReplicatedStorage.XP). That wouldn't run the rest of the script if XP didn't exist in ReplicatedStorage. You can only require things that are module scripts.