What does the ending mean?

The ending as I understand it is full of subtlety and what is unsaid. Her "Bobby?" and walking a few paces behind him is symbolic of her apologising and asking his forgiveness; indicating she is still there for him if he wants her. Her continuing to walk behind him in silence indicated that she is willing to go the distance with him, whatever it entails. They are joined together, for better or worse. She knows him, and accepts him for what he is - and what she has become because of him. She accepts the life they will inevitably have together, of addiction and hardship and crime, and constantly scratching around and debasing themselves for money. In its simplest form, she is demonstrating her continuing love.

His "Well?" is an acceptance of all this. He forgives her, he invites her to walk in step with him. They present a united front to the world. The shot of them walking side by side, him pulling up his hood, is suggestive of them gritting their teeth and walking into the future together - a circular future which can only ever bring more of the same for the both of them. Their bond can be seen either as a very messed up and damaged/damaging kind of love, or of two lost and lonely people bonded only by the shared love of/need for a drug. Either way it is very bleak! And free of any kind of redemption.

Hope that helps!

Great summary. To add. I think it the end is symptomatic of most negative relationships in which both parties are addicted to each other. Hence Helen following or being hooked on Bobby despite all that had happened. Even worse was Bobby allowing his addiction to her back in...'well' and allows her to walk with him.

Bleak