what does "be in the moment" mean?

You ask 2 questions, but only want answer for 1 of them?

No, that's not necessarily what it means. To "be in the moment" means to have one's attention focused on the present and the situation one is in. It means not thinking about other places or events, not thinking about the past or the future--it means thinking only about the present, and the place and event in which one is involved.

It is an expression so consistently used in a meaningless way, however, that it often doesn't mean anything. I haven't seen the motion picture to which you refer, but i wouldn't be surprised to see it being misused in a motion picture. If someone is pointing a gun at you and robbing you, i suspect that you would be totally in the moment. I doubt, however, that you would be happy.

I suspect that in a movie, encouraging one to "be in the moment" means to 'enjoy what is.'

Being excessively concerned about what happened in the past or might happen in the future can rob a person of the enjoyment of the present.

Of course, "just living for now" can be a symptom of stupidity and fecklessness.

Movies are a poor source of wisdom.