A REST client is an application that runs on a computer that allows you to issue HTTP requests against a REST API. Each REST client allows you to control the request type, such as GET, POST, PUT, PATCH, or DELETE, the request content (body), any headers, and deal with the request responses. For example, showing the returned Http response code and any response headers, as well as format the response so it can be easily understood. There are many REST client applications available, the most popular are Postman®, Fiddler®, Advanced REST Client, and Insomnia® client.
Most REST APIs are used for machine-to-machine communication, and a REST client is normally used by developers when debugging or testing features in a REST API. Any REST client application provides the ability to easily interact with a REST API in a more convenient way. They normally record any requests made and their responses, so that you can quickly replay a scenario or compare a response to an earlier one.