A floater is a term for a change in the fluid inside the eye. Two-thirds of your eye is made of a fluid-like a gel called the vitreous. Over time that gel becomes a little runnier, and when it moves and becomes more fluid some fibers stick together and cast a shadow on the back of the eye. They’re a nuisance but they don’t steal our vision. Over time because the fluid is moving around, they eventually will move out of the way. They tend to be a lot more visible against a blue sky or white background.
The floaters that are more of a concern are showers of floaters accompanied by flashing lights. The fluid sits next to the retina, and if it pulls on the retina it can cause a retinal detachment. So any new floater that you notice should be checked out to make sure the retina is not at high risk.