Within the movie, understanding the title is a little...difficult. For clarification, let's read an interview the director did with Grantland:
Grantland: Near the end of the movie, Dane DeHaan's character gets a gun, and the rest of the time you're waiting for the grisly outcome. But it doesn't happen.
Cianfrance: I was thinking a lot about what is this film ultimately about. What’s Jason gonna do with the gun? Is it gonna be a story of vengeance? That would be very satisfying. But I don't feel that in my heart. I have kids and my kids have to watch my movies someday. I don’t want to put that in the world. Is he gonna kill himself? Is it gonna be hopelessness? I don't feel hopeless. I don't wanna put that out in the world. To me it's about forgiveness. I think about America: the legacy of this country being built on brutality and violence and ruthlessness. And now we live domesticated lives, but I don't think that stuff ever goes away. The chickens come home to roost. But the only way to deal with it is to forgive. That's what I would like to put out in the world.
Twice Bradley Cooper is taking into the woods.
The first time by Ray Liotta. We're pretty sure Ray Liotta's going to kill Cooper, right? Liotta is a corrupt cop who has shown flashes of being an asshole. He's menacing as can be. Plus, he's leading Bradley Cooper way off into the woods, alone, after Cooper tried to reveal the corruption charges. Cooper knows exactly what will happen if he goes into the woods. So...he runs away.
The second time is by Dane DeHaan/Jason, the son of the man Cooper had killed. Jason wants to kill Cooper. To avenge his father.
The title then is referring to this idea of choosing forgiveness. The movie ends with Jason heading off into a new life.