I'll throw this out there about the meaning...
When I first watched this movie, I liked it but didn't like the ending. I was like "oh god, not another lazy writer who wants US to finish telling THEIR story to ourselves".
But a day later, the movie kept playing in my mind and I feel the movie is about this...
I think the story is about the woman AND her husband. I think the young guy is a younger, nicer version of her husband and actually may be how he was when they were dating or first married. One thing that lead me to this was the golf. What are the chances of this drifter coming into her home who happens to also be good at golf? Perhaps she was trying to stop him from playing golf because it may have been the first thing that caused her husband to become the man that he is (for instance, golf symbolizing business and affluence that her husband obviously strives for, (the ball hitting the woman in the car to also show how his goals in life has hurt people and have been destrutive)) Then there are other things. Like with the police, when they checked the guy out they said he had a degree, etc. (I also noticed that while this guy was a drifter he drove a nice BMW bike, his character was almost to say there was a balance between the finer things and being humble (i.e. hand washing clothes when you don't have to, yet driving a nice vehicle). And once this guy was locked up, the woman's husband tried to be much nicer but this is what lead me to think that the story is also just as much about him.
So I asked myself, why would the husband be able to interact with the guy? His interaction with this young guy is a war between the mean person he is and perhaps the good person he used to be when he was younger. His current self is so determine to stay on the path to what he thinks life should be like, it drives him to fight the more carefree side of himself. I mean the end is like "I know that nice part of me is somewhere around here threatening to ruin all that I've worked hard to get at his expense."
I mean the zero on the scale thing is pretty straight foward, who they used to be when he was a different person, doesn't exist anymore... If you want to get more silly you could say, when he fixed the scale, he weighed 60 she weighed 40 = 100 which could be taken as together they are 100% or that thier relationship is uneven with him being more important than her.
One could also see that this guy was just in her mind also because he seemed to do some dumb things that if you are a seasoned drifter, you probably wouldn't do. Like, just hanging up the phone on the dead guy's son AND still staying in the apartment like nothings wrong. I mean, I'd like to think that a seasoned drifter would know he should bounce. Or always leaving his bike right outside the houses they break into. I mean really? But if this man is created by this woman's mind, I can see why these mistakes were made because its really her who is making them. Why would she know how to be a good drifter?
I also think that whole prison scene again was back to being directly about the husband. He was trying very hard to cage that part of himself. The nice part of himself or who he used to be. So the nice part of himself, learned to be there stealthfully(and in the end, the wife learned to relate to him as if he were still that nice guy he used to be. i.e. see past the abusive person who he'd become like so many abused women do). The cops were a part of the husband's reality. They kept his nicer self in check.
I also think the couple who's home they went back to and just let her sleep, represents what they could have been. Those two were shown watering or tending to plants when they were shown, representing two nurturing people which would be required for a good relationship.
**On a side note, man he had some raggedy feet!
This is all I have for now, just some thoughts.