The premise of this movie is that this guy can travel backward in time, but only to times and places he's been before. He can only return to an earlier time in his life and re-run it -- but with the knowledge from the present.
At first he only goes back a few minutes. He's socially awkward with girls, and by reliving them eventually learns what to say and do in botched encounters with girls. Later he goes back further and further, to try on different partners or make bigger changes in his life.
Since he can only travel backward, it follows (though not really spelled out in the movie) that he has to live forward from his chosen point of re entry. If he goes back very far, it stands to reason that tiny changes would accumulate so he'd reach a different end point than where he came from, no matter how careful he was to make exactly the same choices every day of his life. At one point he travels back to before his daughter was born, and when he gets back to the present time -- only seconds in the film, overlooking the "jump back, live forward" nature of his time travel -- he discovers his daughter is now a son. The randomness of a sperm fertilizing an egg means the replay of his life results in different children.
Eventually, he learns to live each day fully, with maximum positive energy, and stops time traveling. This is basically the same moral as Groundhog Day.
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