It might be something a casual fan wouldn't understand, but clearly u/RusevReigns is referring to the role 9/11 plays in Bobby Axelrod's backstory and the rich lore of the Billions universe. Understandable that this might go over your head, as to be fair, you have to have a pretty high IQ (or just a lot of spare time) to get all the references in such an intelligently written show as Billions.

/s