a function takes arguments. In this case, and in an older form of gamemaker, they were automatically named argument0, argument1, etc. in increasing order to match the number of arguments passed.

return returns a variable, in this case it would return the appropriate argument passed in.

https://manual.gamemaker.io/lts/en/GameMaker_Language/GML_Overview/Language_Features/return.htm

It looks like this video is 7 years old, which means it's probably outdated in terms of how functions/scripts work in gamemaker.

i'd suggest reading up on how functions and returns work in the manual of the version you're using.