It's part of a big family of jazz chords called altered chords- it almost always precedes a chord a fifth below (you hear there are some other similar dominant-type chords afterwards then it relaxes to D minor at 2:14) It's like a super jazzy dominant seventh chord.
Altered chords have
both (sharp
or flat) 9
and 5.
This particular one is A7 #5 b9. You omit the normal fifth (E) because it clashes with the #5 (F)
Normal A7 = A - C# - E - G
A7b9 (jazzed up A7) = A - C# - E - G - Bb
A7#5 ( another jazzed up A7) = A - C# - F - G
A alt (A altered) = A - C# - F - G - Bb
A alt could also be played as A - C# - Eb - G - Bb (b9 b5)
or as A - C# - F - G - C (#9 #5)
or as A - C# - Eb - G - C (#9 b5)
My favourite shortcut to playing them on piano is to play a Dominant seventh chord with the left hand, but leave out the fifth (so A - C# - G in the left hand). Then using the right hand you play a normal major triad, but starting on either the #5 or b5 so either FAC (F major) in the right hand or Eb G Bb (Eb major) in the right hand.
There's a lot of info there and altered chords are a bit overwhelming at first but once you learn them it's really nice to be able to mash them out quickly. Ask me again if you need anything
