The check Apple and Google run on every app or update before publishing it in their stores, to confirm it follows their guidelines.
Before your app appears on the App Store or Google Play, store teams and automated systems review it. They check that it works, that its description is accurate, that it has a privacy policy, that it asks for permissions for a clear reason and that it follows payment and content rules. The guidelines change from time to time.
Rejection on the first attempt is common and delays launch. Frequent reasons include an app that looks like a thin website wrapper, a demo account that does not work for the reviewer, unjustified permissions or missing information. Read the store publishing guide. Practical tip: read the store's current official guidelines before submitting, prepare a working demo account with clear reviewer instructions, and leave time in your launch plan for a possible rejection and resubmission.