Drop-D Tuning is standard tuning with the 6th string (Low-E string) tuned down a full step.  If tuned properly, the note at the 7th fret of the 6th string should match the open A note on the 5th string.

This tuning is useful for playing power chords because you can barre strings 4, 5, and 6 on any fret and get a power chord rooted on the 6th string.