Kinda hard to describe. For a website you usually have severally programs that do different things. The User interface, which is the website you interact with, the middle teir, which is usually in a strongly typed language like java, golang, or Rust (in the case of lemmy.) It usually sleakes to a database to store user information. Like Postgres or mysql. Then you might have very topic specific programs. Like picture hosting programs. Lemmy had one called pictrs. It's responsible for uploading, shrinking, storing, retrieving pictures people upload. For example, when you upload a photo it makes atleast two copies. One large and one small. The small one is loaded up in the main page so the list loads quicker.

When you enter a address in the address bar, a reverse proxy is often in charge of which service get called. The reverse proxy sits in front of all those programs and coordinates the communication between all of them.

This can be used for load balancing. Large companies especially can't handle millions of people logging into 1 computer. So a reverse proxy can split the connections. Send some here, some there, some over yonder.

Nginx is the most popular reverse proxy. Written in C by Russians. It's really replaced Apache over the last 10 yeats. Cady is a newer but slower one written in golang. It's easier to use I've heard.

Basically a reverse proxy forwards web request to multiple servers or virtual servers.