Reverse proxy

The common but confusing term for a set-up where one web server sits in front of another and forwards requests to it. Typical scenario: Nginx listens at ports 80 and 443 on the server computer and forwards requests to a Python web server running locally at port 8000.

Why? You'd rather have the messy details of talking to the outside world (HTTP/2, TLS, etc.) handled in a dedicated, battle-hardened program like Nginx than in your Python web server. Also, it lets you serve multiple sites from the same host.