A game that has little gameplay and is mostly played to get random item drops which you can sell on a marketplace for minimal real world profit. most of the time the marketplace taxes you and gives a little bit of the money to the creator. the reason it is called a Banana game is because of the most popular example of this is a game called Banana on steam.
Person1: Man I've been making a lot less money from this game then I thought i would.
Person2: Well what did you expect from a Banana game, they are practically a scam.