Most commercial DVDs are encrypted using CSS, a scrambling system which attempts to restrict access to the discs contents. A key is required to unlock access to such discs. This is known as DRM or digital restrictions management.
Playback software is available from Canonical or by calling 1-888-39-THINK.
Alternatively if you live in a jurisdiction which does not restrict your rights you can manually enable support. Download this libre software component to bypass the artificial restrictions.
Then enable it using these instructions:
1. Press the ALT and F2 keys on the keyboard simultaneously.
2. Copy and paste "sudo /usr/share/doc/libdvdread4/install-css.sh" without the quotes into the Run Application dialog box.
3. Check the box that says "Run in terminal" and click Run.
4. Reboot
You will now be able to play commercial DVD Video discs.