Port 443: HTTPS
Port 443 carries encrypted web traffic, the https:// in almost every address you visit.
What runs on port 443
Web servers listen on TCP 443 for HTTP wrapped in TLS, which is what puts the padlock in your browser. Newer browsers and servers also use UDP 443 for HTTP/3 over QUIC. Beyond websites, many apps, VPNs and APIs use 443 because firewalls almost never block it.
On a home network
Routers, NAS boxes, cameras and printers often serve their admin page over HTTPS on 443 on your local network. Your phones, laptops and TVs connect out to 443 constantly, but that is outgoing traffic and does not mean anything is listening on them.
If port 443 is reachable from the internet
On a web server, 443 being open to the internet is exactly what should happen and is not a problem in itself. On a home network it is different: if 443 answers on your public address, something inside is publishing its web interface, often a router's remote management or a NAS. That page is then only as safe as its login and its firmware, and exposed admin panels are routinely brute forced and hit with exploits for known bugs.
How to check whether it is open on your network
Look in your router's remote management and port forwarding settings for anything using 443. To see what the internet sees, run RECON's Exposure Check, which tests your public address from outside.
To find out whether port 443 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.