This is probably a stupid question but.. what is considered a 'win' in the slots? Is it hitting the jackpot? I'm getting this objective constantly with Drill Quests and I have no idea what he means by that because if so, then that's pretty annoying since the chances of you winning are completely random..

The Slots is a book :) I am pretty sure this is what the quest means

Ah, thank you! I actually never thought of that. XD

yes that is what it means. It takes about 50 spins to get it, and it's the win you get with three books across the middle.