Drop D Tuning, DADGBE is an alternate guitar tuning, from the standard E tuning EADGBE.
Drop D tuning is frequently used in heavy metal; guitarists who play heavy metal usually need very fast changes between power chords. The tuning has also been used in many other types of music, including blues, country, folk, and classical.
Drop D allows fingerpickers to play chord shapes higher up the neck while maintaining an alternating bass. The tuning also allows for chords with a root or bass note of D to be played with a D an octave lower than with standard tuning, and allows playing of open D chords to include the fifth and sixth strings for a deeper sound. This can be especially useful for songs in the keys of D major or minor. The tradeoff is the loss of the bass E note in chords or fingerings which the player cannot adjust to include fretting the sixth string at the second fret.
Here is a video with an example of the sound of Drop D tuning.