Short for Network Basic Input/Output System, NetBIOS was developed by IBM, Microsoft, and Sytek. NetBIOS allows computers and applications to communicate with network hardware, and allows data to transmit properly over a network. For example, Microsoft Windows computers that are named in a workgroup and not a domain use NetBIOS names, which are converted to IP addresses.
NetBIOS commonly communicates on ports 137, 138, and 139. If your firewall blocks any of these ports you may get errors when your computer communicates with another computer that utilizes NetBIOS (e.g., Microsoft Windows). To prevent these errors or warnings, you must provide access on these ports.