Welcome to... "Micro-Lesson 291"
The progression begins from the, open 5th-string, "A," and uses a 7th position power-chord across strings; 4, 3 and 2. This chord rings out across measure one and enters into a resolved melodic phrase as measure two appears. The resolution phrase performed in measure two is simple, with it's focus on the, "E," at 3rd string's 9th fret.
In the third measure we get a sustained, "G," chord with the low, "A," 5th string open hovering under it. The open 2nd string (B) rings out on the top end of the, "G Major," harmony.
Measure four drops the power-chord theme down into the 4th string's 3rd position and creates an, "F" power-chord. However, the harmony here is more complex. Continued use of the open 2nd string (B) adds an altered interval into the mix. This altered tone is a, "b5," against the, "F," root. Further interest into an expanded, "F," chord sound brings in a top 1st string, "E," which expands this, "F," chord into the quality of, "Major Seventh."
When performing this high, "E," you can use, "Hybrid Picking," (if flat-picking). If playing fingerstyle, perform the upper, "E," string with either the middle or ring fingers.
Since these shapes in this riff are all based upon the "Power-chord" concept, they will be fairly straight-forward to learn. The power-chord is one of the most well-known and widely used chords on guitar. This should make learning this riff easier for most players.
Take your time with the open string concepts and be sure to allow all of the open strings lot's of space to resonate. Enjoy!
Micro Lesson 291: "A Minor" Open-String Chord Riff