Even on rewatching it really doesn't make sense. Not because it is time travel, but because the story is not fleshed out enough to leave breadcrumbs for the uninitiated (people who have not read the script) to be able to follow it. In the end you have to take the narrator's word for it that the film makes sense.
There is a story here. I personally hope someone else takes the time to tell it well.
With Shane Carruth's Primer, the concept of time travel is pushed as far as it will go. The film is infamous for just how complex its time travel concepts, involving multiple timelines and "doubles" occupying the same time plane as "originals," become to progress its plot. It's lack of traditional exposition is both masterful and infuriating, as it forces you to put the pieces together yourself without any hand-holding. Anyone claiming to completely understand this one after one viewing is a bold faced liar.
It's all so complex, but it's not the point: in fact, it almost feels like Carruth is commenting on the impracticality and incoherence of time travel. The two leads clash with their ideals of time travel usage, one being rash and doing-before-thinking, the other being more hesitant and careful. It's this relationship that drives the movie, and acts as a very interesting character study in the process.
Purely as a film in a technical sense, it's a marvel. Produced for just $7,000 (pocket change for the producers of the "Big Six" movie studios) and nearly a one-man show (Carruth directed, wrote, produced, starred in, edited, and composed), Primer has no reason to look as good as it does. It helps that it's so grounded in reality in terms of production value as well as narrative. The time travel device is nothing flashy, and fits perfectly in the garage workshop that puts Jobs and Wozniak's setup to shame. The camerawork is simple, and doesn't distract us from the story with any camera tricks for the sake of making the film "look cool." It's essentially substance over style, a refreshing change of pace from visual heavy films that lack strong narratives.
With one film, Shane Carruth proves himself to be a force to be reckoned with. Such experimental filmmaking as this is rare, but Carruth has breathed new life into indie filmmaking. It's not even something you have to understand to appreciate: considering just how complex this one is, that's a blessing.