The very rough idea is this:

I have a server with multiple services and only one open port (not counting the SSL port) on my router.

Traffic comes into that one port straight to my server. That server has a reverse proxy installed with a directory of subdomains and associated ports.

It internally routes traffic coming in on my open port to the internal services on the server without having to expose them directly to the internet.

The big advantage is that because it all goes through my server, I can add SSL certs to just that and now all of my subdomain services get the benefits of the SSL cert on the host.

I’m sure there are other uses for reverse proxies, but this is how they work in my setup.