This riff is pretty straightforward. We're basically going to be strumming through some Drop D chords while exclusively using sixteenth notes.
One thing I really want to point out with this, is how much we need to focus on muting unwanted strings. In the video, you might notice that I'm literally strumming all 6 strings the entire time. But each chord only has 2 to 4 strings/notes that should be played. So that means we need to use our fret-hand index finger to mute whatever strings aren't in the chord. Be sure to gently rest your index on the applicable strings so the unwanted notes don't sound.