Well depends what you want explained...the best way is to start the movie at the end.The old Nemo was living in 2092 when Anna in space predicted the big crunch would occur that is why after he died at the end he is shown waking up and walking backwards as everything would revert back to the beginning.Meanwhile even though the old Nemo narrates the story of his life to the interviewer,only one of his lives was real because the movie is based on the premise that choice affects outcome.The old Nemo felt the most crucial moment in his life was when his parents seperated so beyond the choice of which parent he chose to remain with are possible alternative paths his life would take.The reason why the buildings fragmented at the end is because in hindsight it looked as if he was telling a story that had not happened but only as a possible consequence of the choice of the child Nemo, as long as Nemo chose one thing the alternative life would become unreal and in fact annihilated.
That movie for example shows the different lives Nemo could have lived depending on which girl he chose to love.He really loved Anna throughout the movie and the movie shows how his or her life would have been if he had met someone else.In fact he and Anna were mutually in love while his marriage to Elise was problematic throughout because he loved Elise while Elise loved Stephano so it was an empty marriage with Elise , meanwhile his life with Jin was similarly flawed, but the role was reversed Jin loved him while he married her without really loving her(just to get back at Elise's rejection in his teen years).In fact i think in his life with Jin he was wealthy and had everything but was in fact a hitman for hire that is why he died by being killed in a hotel room under a false identity and was only identified as Nemo by Jin in the morgue.
Another way to look at the movie is to see it as a near death experience of 15 year old Nemo after a horrible motorcycle accident.Remember the movie kept going to a scene of him in the hospital heavily injured and comatose.Plus there are about 3/4 scenes depicting an accident one is the obvious one where he falls off his motorcycle at high speed another is when he walks in the middle of the road and sees the headlights of a car about to hit him head on and they become two motorcycle lights on either side of him.In another scene he drives into a lake on his way from work when a bird hits his windshield and lastly when there is an explosion during a traffic jam that kills Elise but Nemo survives.So my view is that the only true accident was the one on the motorcycle as that was the point in life where after his thoughts could have wondered to what would have happened if he had not had the accident and had a relationship with Elise or Jin or even Anna, he also though in the other ways he could have ended up dying.All those other accidents were alternative ways in which he could have died in his other lives ,rather in an untimely fashion involving a motor accident.
The movie generally shows many concepts of the butterfly effect which it shows directly because the theory is that a butterfly flapping wings in say new Zealand can cause a hurrican a month later in America.So for example the brazilian boiling an egg caused it to rain in nemo's home town and blotted out the phone number of Anna that she gave him.It's a movie about life , love, choice , consequence, tentative outcomes , possibilities, what might have been. It just shows the viewer how our lives turn out the way they do and what unique circumstances are common to our own life as well as other peoples lives .For example in the movie nemo feared water and couldn't swim yet he almost drowned in the school pool as a kid, drowned in a car in a lake, got shot dead while in his bathtub which all shows a common thread that while some things in an alternative universe or life would change drastically, others may just change slightly but keep some common unique characteristics one could only describe as part of fate
Anyway that is about it but in no way summarises what a wonderfully complex movie Mr. Nobody is