What is a REST API?

The style is a set of conventions rather than a technology: things in your system are addressed as resources by URL, the HTTP verb says what you want done to them, the status code says what happened, and each request carries everything the server needs so no session state is assumed between calls. Following those conventions is why an unfamiliar developer can usually guess your endpoints correctly.

The hard part arrives once a mobile app is in the stores. Some users will not update for months, so an old client keeps calling the shape of the API you shipped a year ago. That means additive changes wherever possible, explicit versioning when a break is unavoidable, and never quietly removing a field. Web frontends forgive a breaking change because everyone reloads; installed apps do not.