A bit confused on one topic Options

[Adrien.]

So as you look through out the scripts you'll see things like $game_map. How are such calls created? how do you um...create if you will the whole $call_me type of "call."

I cannot grasp this concept for some reason. Using $game_map as an example how do we go from the script called Game_Map to a simple call called $game_map? is it a method? I might be blind But I cant find documentation on the $ part of that call either