Yetris is used in place of any actual activity you are performing to give you an air of elite, indie sophistication by doing something so obscure.
Specifically, it is form of the game tetris played on a yeti. Yetis are notoriously bad for playing tetris on, unless they have cell phones imbedded in them, in which case they are excellent for it.
"So what did you do last night?"
"I played yetris."
"Wow, you must be knowledgable and sophisticated!"
"You want to go out tonight? I can't. I feel like staying home and playing yetris."