Welcome to... "Micro-Lesson 272"
The riff uses primarily "Octave Chords" (two-note chords that are played across skipped strings and are popular with Guitarists like Jimi Hendrix and Carlos Santana). Add to that, this riff also applies 6th intervals, (an inversion of a 3rd interval played between two skipped strings, similar to the octave chord). These intervals are often referred to as "Double-Stops."
In the first measure, the 'octave chord' idea is played from the 5th to 7th positions, (notes D to E). A follow-up idea is performed on the 4th to 2nd strings using the 6th intervals. This phrase is somewhat duplicated on the second measure using the harmony of "D Major" chord. This time the octave chord moves from 3rd to 5th positions, (C to D).
Measure three, is a copy of measure one. However, the fourth measure wraps-up the riff by producing a turnaround idea with the chords of, "G" and the 1st inversion of the "D Major chord," (D/F#).
If performing octaves and 6th intervals is a technique you've gotten used to already, (and you can play them easily), this riff will be a piece of cake for you! However, if these guitar techniques are new, you'll need time to develop the technique involved for using them - primarily muting technique. Take your time, watch the left-hand technique and work slowly until these ideas work out for you. Enjoy!
Micro Lesson 272: "E Minor" Progressive Rock Riff