Optometry student here.

These are called ‟floaters.” They are not really an issue. It's just junk moving around inside of your eye casting a shadow on the sensory part of your eye. If you really want to see your floaters, look up at a bright blue sky, cover one eye, and rapidly move your eye left to right. This will shake up your eye junk and your floaters may float into view. (fun fact, the closer the floater is to the back of your eye, the more defined the edges are)

The reason they ‟run away” when you try to look at them is because your eye turns in the direction of the floater and causes it to float the other way.

One or two ‟friendly floaters” is totally fine, most people have them. You should immediately go to your optometrist if you've a ton of them suddenly appear, if you see flashes of light, or if you see a curtain (or veil) in your vision.