The premise is that we are looking at the home movies of a rather dull young couple living in San Diego. She thinks things are going bump in the night so he rigs up cameras with night vision and sound to prove there's nothing really happening. When they pick up some ambiguous signs that she might be having visitations from a demon, he panics, determines to 'protect his woman' and starts trying to out-macho the demon. Despite being warned not to by a psychic. Duh.
There's a good contrast in the film between the full-colour day time shots, where he is sceptical and relatively up-beat, and the night vision footage, where he is just as spooked as his girlfriend by the odd goings-on. The audience too pick up the day/night contrast in tone, and in the second half of the film, every time the night vision footage started up, people were shifting about, audibly groaning or sucking in breath. The noisy audience made the film more scary.
Not a lot 'happens' compared to many supernatural horror films nowadays. Less is definitely more, to my mind. Possibly for preference I would have pared it down even more, so that it was feasible that all the happenings were in her mind, and she was gradually infecting him with her own insanity. Like The Little Stranger.
The paranormal activity is a little more explicit than that, but the constraints of very low budget mean that a little has to go a long way. The most frightening scenes in the film, apart from perhaps the very end, are when we see her getting up in the night, as if sleepwalking, to stand - staring expressionlessly down at his sleeping form - as the film goes into fast forward and we see the hours ticking by, and she is just standing there, jiggling slightly.