IAP is in app purchase - this is when you purchase something in an app and pay through the app store.
It simplifies payments because typically the user already has their info on the app store, reducing. checkout friction, and you’re paid directly through the app store. Users will also trust IAP over other payment methods. Plus, as LaoEntrepreneur’s problem highlights, Stripe isn’t available in all countries. IAP should solve for this too.
The downside (quite significant) is the fees - which are in the range of 30%.
I should note, that for most apps, you’re legally supposed to do IAP.