REST is also known as Representational State Transfer. A REST service is a simple software functionality that is available over HTTP protocol. It is a lightweight service that is widely available due to the popularity of HTTP protocol.
Sine REST is lightweight; it has very good performance in a software system. It is also one of the foundations for creating highly scalable systems that provide a service to large number of clients.
Another key feature of a REST service is that as long as the interface is kept same, we can change the underlying implementation. E.g. Clients of REST service can keep calling the same service while we change the implementation from php to Java.