What does a ModuleScript do?

I checked out the ROBLOX WIKI, and it said this; 'A ModuleScript is a Script-like object that contains code for a module that can be loaded by calling the require global function with the ModuleScript as an argument. Its code will only be executed once per peer (server/client). This makes it useful for creating libraries of code that are shared between scripts on the same peer.' Hope this helped!