CSS means Content Scramble System, and is the encryption used by a lot of commercial DVD movie disks, although newer DVDs use something different for encryption.
Most hardware DVD players have a CSS decryption module built-in.
The DeCSS is a reverse-engineered code which breaks that encryption. It's available through some software such as MacTheRipper (MTR), and others. Here's a link for MTR: http://www.versiontracker.com/dyn/moreinfo/macosx/22715
Use at your own risk...