2.2 Becoming Familiar with the Common Desktop Environment
2.2.4 Locking the display
Left clicking on the Display Lock button on the Front Panel locks the screen. The backdrop will be blank, by default, and the Password Prompt box will be displayed for only a short time. To bring the password prompt back to the display, move the mouse or press a key on the keyboard. Once the display has been locked, it can only be unlocked by entering the correct password for the user who locked the screen. 

Note - If a user locks the display and has forgotten the password, the root user (superuser) can enter the root password to unlock the display. The Style Manager, discussed later, enables you to choose a screensaver program while the display is locked.