Ubuntu is pre-configured to start the screensaver after five minutes of system inactivity. You have to move your mouse or press a key to exit the screensaver, but the root password is required to disable the screensaver.
If you want to remove this screensaver password on Ubuntu 11.04, then follow these simple steps:
1. Using the dash, search for "screensaver" and open it.
2. Check off "Lock screen when screensaver is active", then click Close
3. From now on, your password will not be asked whenever you exit the screensaver.