Both REST and gRPC are protocols for communication.
REST is a way client-server communication based on HTTP/1 w/ GET, PUT, POST, DELETE, PATCH methods. Roy Fielding introduced with a set of concepts like cachability, stateless behavior etc.
It uses the Protocol Buffers binary format (Protobuf) for data exchange. Also, this architectural style enforces rules that a developer must follow to develop or consume web APIs.
RPC follows a client-response model of communication for designing web APIs that rely on HTTP/2. Hence, gRPC allows streaming communication and serves multiple requests simultaneously. In addition to that, gRPC also supports unary communication similar to REST.