Drop d is a guitar tuning, and is perhaps the most common example today of what is called an alternative tuning for the guitar. To tune a guitar from standard tuning (EADGBE) to drop d, detune the low e string one whole step.
This tuning is very common in today's rock and roll music because it allows low power chords to be played using one finger. Excellent examples are Milquetoast by Helmet and Killing in the Name by Rage Against the Machine.