When set-user identification (setuid) permission is set on an executable file, a process that runs this file is granted access based on the owner of the file (usually root), rather than the user who created the process.
This allows a user to access files and directories that are normally only available to the owner. For example, the setuid permission on the passwd command makes it possible for a user to change passwords, assuming the permissions of the root ID.
You must be extremely careful when enabling Set UID mode, because it can constitute a security risk. For example, a user can gain superuser permission by executing a program that sets the user ID to root.