Reverse proxy
Also called Load balancer, Gateway
A server that stands in front of your real server, takes requests from outside and passes them on. It can handle encryption, share load between copies and cut off slow replies, so "the server" is usually several programs.
When an agent looks for an error in your app's logs and finds nothing, it may be looking in the wrong program. A 502 or 504 often comes from the proxy, and your code may never have seen the request. Naming the layer sends it to the right place.
Say it to your agent
“We're getting 504s from nginx with nothing in the app log. Compare proxy_read_timeout with how long /report really takes and show me both numbers. Then make /report faster or move it to a background job. Don't just raise the timeout.”