Drop D tuning is D A D G B E, from the lowest (6th) string to the highest (1st). Compared with standard tuning, one string changes.
Only the 6th string goes down a whole tone. You get a deep D bass note and one-finger power chords on the lowest three strings, while every other chord shape stays the same.
The open strings ring G6 (with 9 as the lowest note), with the dropped string as a deep root.