Port 1900: SSDP / UPnP
Port 1900 is SSDP, the discovery part of UPnP that devices use to announce themselves and find each other.
What runs on port 1900
UPnP devices send and answer SSDP messages to a multicast address on UDP 1900 to say what they are and where their control page lives. It is how a TV finds a media server and how a game console finds the router to ask for port forwards. Windows, Xbox, PlayStation, smart TVs and many routers use it.
On a home network
Your router, smart TVs, game consoles, media servers, Sonos speakers, Windows PCs and many smart-home gadgets all use SSDP on 1900 inside your network. That is normal.
If port 1900 is reachable from the internet
SSDP should never answer from the internet. Devices that do are used as reflectors in SSDP amplification DDoS attacks, where a small spoofed request produces a much larger response aimed at a victim, and they also leak device details. Some cheap routers have been found answering SSDP on their WAN side.
How to check whether it is open on your network
An outside scan of UDP 1900 on your public address should get no reply. Also check your router's UPnP setting; if you do not need it for gaming or media, turning it off stops devices opening ports on their own.
To find out whether port 1900 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.