Drop D is the easiest alternate tuning there is, and one of the most useful. You change just one string, and suddenly your guitar sounds deeper, power chords take one finger instead of two or three, and you get a big low drone you can ring under almost anything in the key of D. If you've never strayed from standard tuning, this is the perfect first step, and you can be playing in it within a couple of minutes.
In standard tuning, your six open strings are E, A, D, G, B, E, from the thickest string (string 6, nearest your chin) to the thinnest (string 1). In Drop D, only the thickest string changes. It goes down a whole step, from E to D. A whole step is the distance of two frets, so the string now sounds the same pitch it would if you could play "fret minus two" on it.
So your open strings in Drop D are:
Notice you now have two D strings, string 6 and string 4, an octave apart. An octave is the same note name at a higher or lower pitch, like the same word sung by a bass singer and then a tenor.
Tip: tune up to the note if you overshoot. Coming up to pitch helps the string hold its tuning better.
In standard tuning, strings 6 and 5 are a perfect 4th apart, which is five frets. In Drop D, strings 6 and 5 are a perfect 5th apart, seven frets. The rest of the guitar keeps its usual relationships: perfect 4ths between most strings, and the familiar major 3rd (four frets) between G and B.
That one change on the bottom pair is the whole secret. Here's what it gives you:
Strum all six open strings and you'll hear D, A, D, G, B, E. It's not a neat, finished chord; it sounds open and a little unresolved, with a strong low D underneath. Interesting, but not the main event.
Now strum only the bottom three strings: D, A, D. That's a clean, solid D power chord, the root, the 5th and the octave. Listen to how wide and stable it sounds. This is the sound Drop D is famous for.
Try the one-finger trick at a few frets on strings 6, 5 and 4:
Bonus: play your normal open D chord shape on the top four strings and strum all six. The low D and A now belong to the chord, so you get a huge, full D major.
Here's the good news: strings 1 through 5 are exactly as you know them. Every scale shape and chord shape that avoids string 6 works the same.
On string 6, every note is now two frets higher than it was. A G that lived at fret 3 now lives at fret 5. An E that was the open string is now at fret 2. So when a scale pattern or a chord's bass note uses string 6, slide that note up two frets and leave everything else alone.
Chords with their root on string 6, like the barre shapes you might know, need reworking, which is why many players simply switch to the one-finger power chords or play those chords from string 5 instead. Explore it on the interactive fretboard: watch how only the bottom row of notes moves.
Let the open low D ring and play a simple riff: strike the bottom three strings open, then fret 3, then fret 5, then back to open. Keep it slow and steady, mute the higher strings lightly with the underside of your fretting fingers, and listen for that big, even power chord sound each time. Once it feels comfortable, add a quick open D between each chord. Give it five minutes a day for a week, and Drop D will feel like home.
More on the instrument itself: the 6-String Guitar guide.