Server knocker is an application that can listen to a port and proxy packets to another application.
The proxy will also wait for a idle timeout. If it doesn't get any new packets in this time, it will kill the child application.
Then, if it receives a new packet, it will spawn the child application again so it can continue proxying packets to it.
If you know serverless services like AWS Lambda, you can think of this in a similar way. It aims to reduce resource usage by shutting down the idle server and spinning it up when a net request arives.
The goal of server knocker is to save resources when running some expensive application that is not used often. For example, a game server to play with friends every once in a while without wasting resources in the server.
Think a minecraft server that can automatically shut down during the night, and spin up automatically when someone tries to join in.
Use server-knocker --help for detailed up-to-date information.