This riff is pretty standard and is in drop D tuning. In drop D tuning the low E string is tuned down one step to D. This enables the guitarist to play power chords with a single finger. If your favorite
band uses drop D tuning and you want to experiment with this tuning, by all means, do so. Whatever you do, don’t learn the guitar in drop D tuning-you’ll get all mixed up.
To tune your guitar to drop D tuning, start in standard tuning. Then you’ll want to tune your low E string down D (we drop it down one step, hence the name). See the picture
below. You can use the audio clip below to tune your low E string to D for drop D tuning.