They are caused by an irregularity in the vitreous ‘gel’ that fills your eye. The irregularity casts a shadow onto your retina, blocking small areas of your vision, which you perceive as floaters. Floaters occur naturally as the vitreous gel within your eye thickens or shrinks with age, causing clumps or strands to form. In most cases, floaters are completely harmless, if a little irritating.
In some cases, the shrinking vitreous gel can pull on the retina (the focusing surface at the back of your eye). If this occurs, it can cause a tear in the retina or even detach the retina. This is usually accompanied by a sudden increase in floaters, and sometimes flashes in your vision as well. If you encounter any of these symptoms see an eye specialist immediately.