The user ID or UID is what the system really uses to enforce permissions
on files and processes. If two users have the same UID, they will be able
to access each others files - so every user should have a unique UID.
When creating a new user, Webmin will fill this field with a UID that is
not in use by any other user.
The root user always has a UID of 0. If you create another user
with UID 0, that user will have the same powers as root.
If you are editing an existing user and change the UID, the
Change user ID on files? option determines which files
(if any) have their ownership changed.