A URI is a string of characters used to identify a resource over the web. In simple words, the URI in a RESTful web service is a hyperlink to a resource, and it is the only means for clients and servers to exchange representations.
The client uses a URI to locate the resources over web, sends a request to the server, and reads the response. In a RESTful system, the URI is not meant to change over time as it may break the contract between the client and server. More importantly, even if the underlying infrastructure or hardware changes (for example, swapping the database servers) for a server hosting the REST APIs, the URIs for the resources are expected to remain the same as long as the web service is up and running.