Bickle Fake-Shooting Himself with His Fingers after Cops Arrive
After Travis has killed three people, leaving Iris free to abandon her life as a twelve-year-old prostitute, he tries to kill himself, but—whoops—he's out of ammo. Wounded, he sits down on the couch and waits for the cops to arrive. When they do, he pretends to shoot himself in the head with his bloody fingers. The cops just look on, silently.
This isn't going to be much of a revelation, but this is meant to demonstrate that Travis is—crazy. (Shocker, we know!) We assume, since the cops see this, that he's going to go to prison or a mental hospital or something.
However, he… doesn't. We go from seeing Travis pretending to shoot himself to seeing Travis hailed as a hero, a noble vigilante. The image helps heighten the irony of this transition.