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, you will have the following options:

Automatic
  The UID will be assigned automatically by Webmin, based on those currently in use.
Calculated
  The UID will be computed based on the username, using a checksum algorithm.
Entered
  The final option allows you to enter whatever UID you like.

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.