A reverse proxy is a server that sits in front of your real servers. Visitors talk to it; it forwards requests to the machines behind it and returns their answers. The outside world only ever sees the proxy.
That position makes it the natural home for cross-cutting work: terminating TLS, blocking malicious traffic, caching common responses, and spreading load across backends. Nginx, Caddy and cloud load balancers all play this role, most production websites you use today have one in front.