At first it looks like to be the old man with memory issues trying to remember his past. The doctor uses hypnosis on him and so forth. It seems he's delusional, imagining things that didn't happen. Maybe he's imagining what might had happened over failures in his life if he had made the right choice or avoided bad lucks. But as the movie progresses we see that he's depicting multiple timelines, as when the journalist says he can't have had children and not have them together.

Some ppl, trying to make sense on the movie, thinks it's all the imagination of the boy, that imagines it all on a fraction of a second while trying to make the choice. But it's hardly believable that a 9 years old boy would be able to imagine mature situations like living with his girlfriend, marital conflicts or even getting married.

This movie just depicts fictionally (of course) multiple timeline branches that might happen (and that might happen in parallel on a multiverse) based on a few big choices he might make in his life.

We have 2 opposite points. Him as a child on the train station having to think/predict his whole life future if he stays with his father or goes with his mother. And him as an old man on the far future on the merge of dying, trying to remember his past. In both points, he gets confused looking on all timeline branches binding together.

The movie shows a bit of modern quantum physics related to time, it talks about the string theory and entropy. Many scientists try to understand why time flows in only 1 way, if according to modern physics it's just another dimension. Why can't we turn back in time? Why can't we remember the future?

On spacial dimensions, we can walk on both ways. In fact, on most part of mathematics applied to physics, all actions may happen on both ways in time. If we calculate negative time flow, actions happen on the mirrored way of positive time flow. As a lot of physics was first understood mathematically to then proven in reality, why doesn't time flow in both ways?

One of the few cases that we depict where actions don't go in both ways is on entropy. On it, what happens can't be reverted. It shows examples of smoke leaving cigarrete, substances being mixed, death and decomposition. Their opposite actions are never seen in reality, and they are the few situations where we can actually see time flowing on a single way. So, we have the hypothesis that the whole world is decomposing or losing order as it expands.

It also shows the Big Crunch and the guess that, if gravity is able to slow universe expansion until it stops and starts contracting, would time also flow backwards and all that happened while it was expanding would turn back as it contracts. Sadly it's alrdy proven that the Big Crunch won't happen, because the universe expansion is accelerating instead of slowing down. We still don't know why it's happening, and we call the energy that's consumed for this acceleration to happen as Dark Energy.