RPC
In plain terms
Calling a function on a remote server as if it were local.
Going deeper
Remote Procedure Call abstracts network communication so invoking a remote service looks like a normal function call. It hides the messaging plumbing.
Under the hood
gRPC and others add efficient serialization and contracts; the abstraction can hide network failure realities.
Examples
gRPC, Thrift, internal microservice calls, JSON-RPC.
Think of it like...
Like dialing a coworker and having them do a task as if you'd asked them across the desk.