When I finished watching Primer, I couldn't even hope to understand what I had just saw. Being the typical person who wanted to understand it, I read it on wikipedia, and am watching it a second time while I write this now. The basis of this movie is that four guys make a box, it started out by growing enough bacteria of some sort in 5 days that professional labs can grow in 6 years. I'm not entirely sure how, but they learn that when they go into the box, a double of them leaves the box. Since they are in the box, they control the double. What they have done, is go back for a certain amount of time, and be the double to get a more desirable outcome then what originally happened.
They've pretty much gone and done this a bunch of times, and as their understanding of the box grows, they venture out to change more.
That's really all I can think of saying without actually ruining anything. If anyone in the comments has more to add, feel free to do so.
That being said, I thought it was a very good movie despite my somewhat lack of understanding. The fact that it was made on an under 10,000 budget adds to this, and I do think anyone who hasn't yet should see it