Welcome to... "Micro-Lesson 263"
This technique involves the thumb of the fretting-hand, hooked over the top of the guitars fingerboard. When in this position, the pad of the left-hand's thumb is able to reach over the top of the neck and actually fret-out the lowest strings note. It allows the rest of the fingers of the fretting hand to grab more notes in generally a more controlled manner. Several rock players use this, "Thumb in the Bass" technique, (most notably Jimi Hendrix).
In this riff, we play all bass-notes on the 6th string with the thumb in the bass technique. The other fingers simultaneously fret-out the upper tones on the 4th and 3rd strings.
The harmony of this riff operates around the key of "B Minor" and is generated using smaller two-notes chords under the 6th string. The harmonies involving the VI (G), and VII (A), chord play against the Tonic chord (Bm) throughout the first three measures. However, there is a separate turnaround section in where a V-chord of, "F#m," is applied to pull the progression back to the top.
Do your best to play the progression with the thumb in the bass technique. But, be advised however, that not all guitar players are able to play in this way. A small percentage of players have serious limitations with this technique. In those cases, guitarists will need to fret the bass-note of their chords in another way. Have fun learning the riff!
Micro Lesson 263: "B Minor" Pop-Rock Riff