Remember that gamer comic I used to have? Me neither. Anyway, here's another one of them, this time using the old VG Cats approach of casting the player as the protagonist.

Undertale does a good job of explaining almost all of the video game mechanics it uses from an in-universe perspective. One exception is why you always end up with some money when you "win" a fight, even if you didn't kill anyone. Couple this with the mechanic that most fights can be resolved "peacefully" by attacking until they're almost dead and then letting them go, and it suddenly becomes a game about beating people up and taking their lunch money.