grpcurl is a command-line tool that lets you interact with gRPC servers. It’s basically curl for gRPC servers.
The main purpose for this tool is to invoke RPC methods on a gRPC server from the command-line. gRPC servers use a binary encoding on the wire (protocol buffers, or „protobufs“ for short). So they are basically impossible to interact with using regular curl (and older versions of curl that do not support HTTP/2 are of course non-starters). This program accepts messages using JSON encoding, which is much more friendly for both humans and scripts.