Drop D Tuning is commonly used in Nu-Metal. All you do is tune down your low E String from an E to a D (1 Tone - 2 Frets). So where you did have the standard E A D G B E, you now have D A D G B E.

One reason artists do this is to make some chords easier. For example, if you have the normal G5 Chord, you can make this much easier by tuning down to Drop D Tuning...

e|-----|
B|-----|
G|-----|
D|--5--|
A|--5--|
E|--3--|

Here is the standard G5 Chord. If you tune down your low E String to D, the pitch will decrease by 1 tone (2 Frets) so you have to raise you are playing by 1 Tone (2 Frets) so you move from the 3rd Fret to the 5th Fret. This is how it will look...

e|-----| B|-----| G|-----| D|--5--| A|--5--| D|--5--|

Here is the G5 Chord in Drop D Tuning. If you barre all of the strings shown with your index finger*, the chord will be much easier to perform. Now if you play the bottom three strings with no frets fretted, you will have the low D5 chord, regularly used in nu metal. Below is an example of this...

e|--------------------|
B|--------------------|
G|--------------------|
D|--3----0----5----7--|
A|--3----0----5----7--|
D|--3----0----5----7--|
Tool - The Patient

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*For more information on how to barre a fret, see Barre Chords.