Drop D Tuning also known as DADGBE has become popular in modern rock and metal styles because it lets you play power-chords more easily. Drop D is also useful for styles such as folk and country.
Tommy Emmanuel use this type of tuning alot in his songs, so it's a need to know if you want to learn and understand his playing. Also Chet Atkins and Jerry Reed songs use this tune.
If you have watched Chet Atkins play the song The Entertainer, then you will hear how Drop D Tuning sound. You can go and see the video here. In the post I explained how to read guitar tabs for beginners. And also used the beginning of the song as an example
Now like I said only one string is tuned down or "dropped down" to get Drop D Tuning. And this is what must be done. You're lowest string the sixth E string (this is the thickest string on the guitar) Or if you are looking down on the strings when you are holding you're guitar. Then it's the first string on top you see. That's the E string you will need to tune.
Note that an guitar have two E strings. The highest (thinnest) and sixth (thickest). If you were to play both at once. They should sound the same, if you have you're guitar in tune that is.
The way I usually tune my guitar into Drop D Tuning is by using the 4th string that's also a D as a reference. So go ahead and play you're lowest string and the 4th string at the same time. And then tune down you're lowest string so that both strings sound the same. Usually you only need to twist the tuner one whole turn and you would be in Drop D Tuning.
Now to check that everything is in order. You can play a D chord, because you have tuned down you're string it should now also sound the same as you're D chord.
And that's it. Here are some chords for you to try and play in this new Drop D Tuning.