Linux and Windows each use a different ID system to identify groups and users.

Linux assigns unique GID and UID numbers to groups and users, whereas Windows uses a Security Identifier values (SID) for each user and group.

The winbind service maintains the necessary mapping of each Linux GID and UID to its corresponding Windows SID. However, you're responsible for specifying which of the available mapping methods, or backends as they're called in Samba, to use for this mapping.