Warning - This is illegal in many countries, including the United States. Proceed at your own risk.
(In the United States, the law prohibiting the bypassing of DRM is the Digital Millennium Copyright Act, or DMCA.)
By default, Trisquel is unable to play many DVDs because it lacks a library to bypass the CSS (Content Scrambling System) DRM (Digital Restrictions Management) algorithm that prevents access to encrypted DVDs. You need a simple free software library called libdvdcss, which bypasses the encryption, includes an API to access DVDs as a block device, and ignores region codes.
Installation
libdvdcss can be installed via multiple methods.