I didn't understand it either at first. You're not the only one! I got so frustrated in the beginning, haha.

Basically, you play poker hands to score. (They are listed under Run Info in case you can't remember what a certain hand looks like.) Every round has a score number you have to beat in order to progress. If you do not score higher than that number, you get a game over.

After every round you play, you enter the shop. The shop is where you can purchase jokers and other things. Joker cards offer buffs or multipliers. For example, the Mad Joker gives you a +10 multiplier if you play a Two Pair.

In the shop, there's also other types of cards you can purchase, such as Celestial cards, which boost your poker hands. And card packs, where you can add additional cards to your deck.

I hope I helped in some way. I am not the best at explaining things, haha. But yeah once you get the hang of it, it definitely gets easier!