From Wikipedia: Binaural beats are auditory processing artifacts, or apparent sounds, the perception of which arises in the brain independent of physical stimuli.

In some areas of psychology, there is interest in what is called “hemispheric synchronization”.  The idea is that your mood (wanting to be more relaxed, more focused, more creative, etc.) can be influenced by listening to binaural beats.  The sound is put out at certain frequencies (depending on the mood you want) and is done at a level that is physically indistinguishable to people, but is still present.  Lowering the brain frequency relaxes, raising the frequency increases focus.  Binaural beats work best with stereo head phones.

A website that allows you to listen to binaural beats (for free) is I-Dose.  You can choose from many differnt types of sounds, including some for meditation and energy boost.