The XML-RPC network protocol is popular, small, and lightweight. It’s designed for clients to make function calls to a server and receive the results in a simple and language-independent manner.
Recently developed servers will probably offer remote procedure calls based on more modern technology, for example gRPC. However it’s very possible you will still need to write or support an XML-RPC client to access an existing service.
I hope these notes will be useful enough to get you started if you have never used XML-RPC before.