A good ELI5 explanation for a reverse proxy server is to think of it as a single point of entrance for your network as opposed to multiple points of entrance. It checks requests coming in through ports 80/443 - or whatever port you might specify differently - and directs those reuqests to the correct server such as Petio, Radarr or Sonarr. The reason why this is more secure is because rather than randomly punching holes in your firewall for each service, you are only forced to open up 1 or 2 ports that handle all the traffic.