Ok, I'm not sure about this, but looking at the explanation it looks like something like ngrok[0], serveo[1] or Argo Tunnel[2].
So basically it's a program for server & client that allows exposing a local server to the net (usually) for development purposes.
Install mr2 on the server & client, you can use the mr2 program in the client to expose the web server running in the client through the server (with mr2 installed).
[2] https://developers.cloudflare.com/argo-tunnel/quickstart/
Just to add to your list, I've been using frp[0] for the same purpose.