Port 8443: HTTPS alternate
Port 8443 is the common alternate port for HTTPS admin pages and web apps.
What runs on port 8443
8443 is to 443 what 8080 is to 80: a second, unprivileged port for encrypted web traffic. Apache Tomcat, Plesk and the Ubiquiti UniFi Network controller use it by default. Vendors often put an admin interface on 8443 so it does not collide with a website on 443.
On a home network
Routers with HTTPS remote management, UniFi controllers, Chromecast and Google Cast devices, some NAS boxes and security appliances use 8443.
If port 8443 is reachable from the internet
On a hosted server running a real web app, 8443 open is often fine. On a home or small-office network it usually means a device's admin page is reachable from anywhere, and encrypted or not, that page is exposed to password guessing and to exploits for whatever firmware bugs the device has. Keep admin interfaces behind a VPN.
How to check whether it is open on your network
Check your router's remote management settings and port forwarding list for 8443. RECON's Exposure Check shows whether it answers from outside.
To find out whether port 8443 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.