Their song “Testify” is in drop D tuning. In drop D tuning the low E string is tuned down one whole step to D. This enables the guitarist to play power chords with a single finger.
To tune your guitar to drop D tuning, first make sure you tune to standard tuning. Then you’ll want to tune your low E string down two steps to 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.
Here’s what it sounds like looped at the normal tempo of 117 bpm:
That first chord shape is a seventh sharp ninth chord. It’s a chord shape made popular by Jimi Hendrix in the song Purple Haze.