With gRPC, you define your endpoints and request/response messages, and client libraries in many languages are auto generated for you.
If you're interested, you can check out this intro video:
https://www.youtube.com/watch?v=RoXT_Rkg8LA

Someone also posted this about it on r/monero which I think does a good breakdown of pro/con of using gRPC:
https://old.reddit.com/r/Monero/comments/l6fxkl/gauging_interest_grpc_api/