Explain it?
If you slap your thumb on a fret hard it produces percussive note. If the right and he's simulating a kick drum with his thumb slapping against the body of the guitar and the slap of each note produces a the rest of the "beat" as well as the bass tones. In the left hand he's playing another single line melody slapping his fingers against the frets.
It takes practice to do what he's doing, but if you're an experienced bassist it's not hard to do.
My boy Roland Guerin is famous for his slap technique on upright.