Cross-platform graphical remote access, sharing the actual screen of the remote machine rather than starting a separate session. Each additional display adds one to the port number, so a second is on 5901, a third on 5902, and so on.
Often unencrypted, and its password scheme is weak and limited in length. Whole search engines exist that index open VNC screens. Tunnel it over SSH or a VPN.
Open VNC is indexed by public search engines that catalogue exposed screens. Close it and tunnel over SSH instead; its own password scheme is not strong enough to rely on.
The port scanner will tell you whether 5900 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.