Or just think of it as a specification for a standardized way to make web API requests. The web service will be designed to recognize certain URI formats in combination with different HTTP method types. Any request that matches one of those URI format/method combinations will be routed by the server-side code to the applicable method/function/whatever. That web app will then need to publish its API in terms of what URIs and HTTP methods are used to do what.
Or...keep searching for tutorials, because I'm probably doing a lousy job of trying to describe it in 100 words or less.