The older transport for Windows file and printer sharing, carrying SMB over a NetBIOS session layer. Modern Windows runs SMB directly over TCP on port 445 instead, but 139 persists on older systems and on Samba servers configured for compatibility.
Same rule as 137: local networks only. Exposed NetBIOS has been a reliable source of information disclosure for twenty-five years.
As with 137, this belongs inside the network only. If it is reachable externally, treat the exposure as having already been indexed by scanners.
The port scanner will tell you whether 139 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.