Windows file and printer sharing, running directly over TCP without the older NetBIOS session layer. It is how mapped drives, shared folders and domain group policy all move data, and Samba uses it to offer the same service from Linux.
One of the most dangerous ports to expose. WannaCry and NotPetya both spread through SMB. It should never be reachable from the internet, and many ISPs block it outright.
Worth knowing. If you need file access across the internet, put it behind a VPN. There is no configuration of public SMB that is worth the risk.
An open 445 facing the internet is an emergency rather than a to-do item. Close it now, then check for signs of compromise: automated scanning finds it within minutes and the exploits against it are fully weaponised.
The port scanner will tell you whether 445 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.