Games have rules. They set the boundaries of what can and can not be done.
In games like chess, Magic the Gathering and Ticket to Ride; if it isn’t in the rules, you can’t do it.
Dave Arneson and the group of players in the Twin Cities discovered a type of game where you can do things that are not explicitly stated in the rules.
One way to think about how RPG’s are different from other types of games is that they are games that do not have rules for everything you can do in the game.