In App purchases ( IAP's) come in two main types:
1. Payment which permanently gives you something, maybe no ads or more levels etc
2. consumable items which you need to keep buying over and over, usually gems or other items that help the game.
Also sometimes they can be just a tip for the developer who is giving you a free app. Sometimes called 'by me a coffee'
Also be aware of subscriptions: Apps get you to buy a recurring subscription which might be monthly or yearly. It's easy to start these going then forget you have them.