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.
Mine often become visible to me shortly before experiencing a migraine. Is there a reason for this? I've told my doctor, but they didn't seem to react to this information much, just said it can happen.
Not sure about seeing floaters before migraines, but what you see could be migraine auras. I have them and they affect my field of vision prior to a migraine. I'll see flashy/persistent "dots" or completely be unable to focus on certain things, like I have a blind spot. Hate it, but it at least gives me time to grab meds before the pain sets in. Usually happens ~20-60 minutes before onset.