In the early 50s, Red came by Henry Morgan's apartment - having learned that he's a doctor - to get his busted hand fixed on short notice. Abraham, who was learning to play the piano with sheet music at the time warmed up to him quickly and soon, they were discussing the delights of jazz music and how it is different from the classical variety Henry was teaching him.
During subsequent visits, Red taught Abraham about how jazz comes from the soul. How the notes are unimportant compared to how your emotions matter to the utterance of the music, as they must guide one's hand. Abraham quickly picks up on this and becomes a natural.
It is only in later years (present time during the show) that Abraham relays this information to Henry, who witnessed only some of it back in the day.
"Just forget about the notes, jazz is about how you feel." - Red to Abraham.