Drop D tuning is when you make your Low E string go down a whole step to a D note. This tuning is a very popular tuning for 6-string guitarists who want to get a lower end with their guitar. I personally love this tuning and use it often in my songwriting.
In order to tune your Low E string to a D note, first play your actual D string (the 4th string) as a reference. Once you hear that, loosen the Low E string tuning knob until the Low E string matches the note you're playing on your D string. Pick the D string and Low E string back and forth while you're doing this. You'll only need to go down a whole step, so you won't have to loosen the knob too much. Once in tune, you should be able to pluck both the Low E string and the D string and both should play a smooth D note.
Also note that the Low E string D note will be an octave lower than the actual D string, so they will sound different in pitch, but they are still both a D note.