It's a Elixir CLI tool that runs an echo server which can be tested using maelstrom.
Install Elixir on your system and follow maelstrom's instructions on how to get ready for testing.
Run this from the root directory of this repository:
mix escript.build
This will generate an echo_server binary which can then be used by maelstrom. Copy this binary to your local maelstrom installation directory and run:
./maelstrom test -w echo --bin echo_server --time-limit 5