SMB stands for Server Message Block (SMB) provides redirector client to server communication on Microsoft networks. SMB is a protocol used for sharing network resources such as files, printers, named pipes for interprocess communications, and other communications equipment sharing.
SMB clients send requests for a service and the service responds to those requests.
SMB provides for two types of security and one or the other is used. It provides for share level security or user level security. Share level security is based on the permissions set at the time any share is set up. The share is set up when an administrator or user decides to share a resource such as a file or a printer. User level security works for Windows domains and the protection is applied to individual resources such as files or printers. The user must be authenticated on the domain and have permission to perform the type of function they are attempting to do on a resource such as reading a file.
A page that explains some more details about SMB is at What is SMB.