So you’ve got something on your computer that you don’t want anyone else to see. To this effect, you’ve encrypted the hard drive. But then you’re put into a situation where an official requires that you unlock the computer so they can inspect the contents (this could happen at a border crossing, for example). That’s where Plausible Deniability comes into play. It’s a feature of TrueCrypt, where you have two hidden encrypted volumes on the same disk partition, and the password you enter determines which one you actually see. So you enter the decoy password, and it unlocks the decoy partition which contains no sensitive files. The other hidden partition appears to be empty space containing nothing but random data. Note that this probably won’t prevent a computer forensics expert from realizing that you have a hidden partition, but the casual observer will probably be fooled.