Representational State Transfer - a set of rules for building web APIs that are simple and scalable. Like having a standardized menu format that all restaurants can use.
Mobile apps use REST APIs to get data from servers, such as retrieving user profiles or posting photos.
REST is an API architectural style (a design approach), not a single cloud service. All major clouds provide services (API gateways, serverless, load balancers) that can host, secure, and scale RESTful APIs.