It's where the server has a proxy that can fetch resources from different places. E.g. ServerA (proxy) get a request for /something, but then fetches it from ServerB (web server) and returns it to the client. It proxies the response to the client.
The comparison is a forward proxy, which the client has, and a reverse proxy is where it's on the server side.
Reverse proxies also often provide optional benefits like...
Load balancing, not having to run the webserver as root to bind to 80/443, caching responses, buffering, rate limiting, injecting headers, serving static assets, etc.
The comparison is a forward proxy, which the client has, and a reverse proxy is where it's on the server side.