The dominant ninth chord extends the dominant seventh chord by a major ninth. It has been found on the fifth degree as dominant since the Romantic period, but has become independent in the course of music history. Thus, especially in blues and funk, it takes over the function of the first degree as tonic and does not have to be resolved.
In the video, the guitarist demonstrates all inversions of the ninth chord over a static jamtrack in A mixolydian. Typical for the genre are also the chromatic slides one semitone lower or higher into the chord fingering – that's really funky!