You tap on an egg to hatch it, but you can't over-tap else the monster runs off. Like say, for one egg it just needs 30 taps but you accidentally tap 31, that monster is gone.
You hatch eggs for monsters which you then fight against wild monsters. I thought maybe this meant you can train your monster, but instead it has only three hearts and if it loses those hearts that monster is gone for good. So you need to hatch more monsters.
The slider is REALLY fast, so you can never adequately time your tap for battles, so it's mostly up to chance if you can win any of the battles.
The monsters all look different but there's really no fighting difference between them. The slider randomly changes shape but never speed. If there was a way to train monsters so they don't feel so expendable or maybe buy upgrades to slow down the slider or power-up your taps, it'd make the game feel like a game instead of a waste of time.
The in-app purchases are for buying "rare" eggs. I'm not sure what you get from them or how effective the "rare" monsters are since I'm not going to spend any extra money on this game.
I'd say wait until the developers make the app free, otherwise don't bother with paying for it.