They're little bits of debris and fibrous points in the gel matrix of your eye. They don't go away ever, but they float to a part of your eye you don't pay as much attention to. They tend to "go away" in people who consistently sleep on the same side. If you're a back sleeper, or move around a lot, they tend to stick near your macula (your main focal point).

When I run ocular scans on people I can see them drifting around. They're not a problem unless they're dense or dark, then they might be blood due to a retinal tear or diabetic retinopathy.