Welcome to... "Micro-Lesson 236"
The riff functions from a fifth-chord harmony with the chord's roots based primarily off of the 6th and 5th strings with the power chords covering the harmony of the piece.
The riff begins from an open, "E," 6th string and uses the power chord of, "E5," at the 7th position of the 5th string as it's focus. Within that same measure, the "E Minor Pentatonic," scale is used to bring in measure two with a riff.
In measure two, the "D5 and A5" power chords enter in our underlying harmony while a "D maj." to an "A Major" Pentatonic phrase is played within the harmony. In measure three, the idea from measure one returns. It brings in a new phrase in measure four that covers two-note power chords of "G5" to "F#5" to "D5."
There is a brief turn-around lick in measure four that will take us back to loop the part is desired. The part may also be omitted to end the riff upon the tonal center chord of "E5" or even possibly "E minor."
When learning this riff, commit the parts to memory prior to building speed. The technique required to play the riff is not overly difficult. But, some areas will require more attention than others for obtaining a smooth flowing sound. Enjoy!
Micro Lesson 236: "E Minor" Classic Rock Riff