The reverse shell client is a Python based alternative for a netcat reverse shell listener (nc -lvp <port>).
The default reverse shell client (rsh-client.py) ships with lots of features such as uploading, downloading, executing and editing files on the (unix based) remote host you managed to get a shell on.
You can also download the standalone version (rsh-client-standalone.py), which is simply a smaller version without the custom commands, mostly useful for local privelege escalation when wanting to get a shell as another process when netcat is not available on the system.