Drop D is an alterate tuning on guitar that is popular in rock, metal, and even fingerpicking. This tuning is accomplished by tuning the sixth string down two steps from E to D so the strings on the guitar become D-A-D-G-B-E.
In Drop D tuning, a power chord (also called a fifth chord) can be played on the lowest three strings with one finger on the same fret (like 3-3-3-X-X-X). This makes it easy to play fast chord changes and to turn riffs into chords for rock and metal.
Drop D also lends itself fingerpicking. With two open D strings, you can play alternate bass lines relatively easily while still having access to familiar chords on the top five strings.