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.

Think thry never go away but your brain ignores them like it ignores your nose (close one eye)

Them going away is probably an illusion, probably a result of sensory adaptation.