The first thing you should know about this movie: it's totally incomprehensible. Many admirable movies ask you to "fill in the blanks", and provide hints for you to do so. In Primer, the blanks you are expected to fill in might be bigger than the movie itself.
But here's the general idea: four friends are (literally) trying to build a business in a garage, doing research, casting around for something they might wangle into a marketable product. So far, they've failed, and it's beginning to strain their professional relationship. Two of the guys, Abe and Aaron, start work on a box gizmo that's supposed to block out external fields. But (surprisingly) a Weeble they insert in the box comes out covered in goo; it turns out to be organic buildup that would ordinarily take much longer to happen. Gradually, Abe and Aaron come to realize that they've constructed a sort of time machine.
So they stick the whole thing in a Delorean, and…
No, they don't do that. Because the box works in a very specific way: at 5:55am, you set the machine to turn on (say) at 6am, and depart. Later, you return, enter the (running) box and wait. Eventually, you get out, and voila, it's 6am! Which allows you to (say) buy stocks that you know are going to skyrocket that day, or engage in other paradoxical behavior.
Speaking of paradoxes, Abe and Aaron aren't actually sure how to avoid them. And can they really trust each other to use the box only in ways they've agreed upon? (Hint: no.) Soon it becomes apparent that something's wrong in the timeline: who did what and how?
The movie was made for $7000, so there are zero special effects, certainly no big names. The characters talk (pretty much) like technogeeks, in a shorthand that means much to them, but leaves the rest of us guessing. Still, I liked it. There are websites dedicated to explaining exactly what happened, the events that stitch together the parts of the plot that actually appear onscreen. If you decide to explore, Wikipedia is probably a good place to start, but I'd suggest waiting until you watch it (at least) once.