Microsoft Knowledge Base Article 243330 contains the following summary:
A security identifier (SID) is a unique value of variable length that is used to identify a security principal or security group in Windows operating systems.
Well-known SIDs are a group of SIDs that identify generic users or generic groups. Their values remain constant across all operating systems.
This information is useful for troubleshooting issues involving security. It is also useful for potential display problems that may be seen in the ACL editor. A SID may be displayed in the ACL editor instead of the user or group name.