If you’re new to playing in drop D, let me first explain what drop D tuning is.
Drop D tuning is when you tune the low E a whole tone lower to a D. In other words, you’re “dropping” the tuning of the low E down to a low D.
That’s where the “drop” term comes from.
Even though this technically makes the string a low D, it’s usually still referred to as the low E string.
When you tune the low E string to a D – it allows you to play power chords (3 note chords made up of a 1, 5 and another 1) using the low E, A and D strings on the same frets.
This allows you to barre these power chord shapes with single fingers, which in turn – allows you to play them with techniques that otherwise might be too tricky for regular power chords; hammer-ons, pull-offs, bends etc.