Floaters are generally caused by some congealing, or "clumping" of some of the vitreous humour (the gel-like substance making up the inner part of the eye between the lens and the retina). This tends to happen as we age. It can also happen due to the separation of some of the vitreous from the retina (known as posterior vitreous detachment).
Most people get used to floaters, and they tend to settle over time and become less noticeable. Generally floaters do not indicate there's anything wrong with your eye, unless you suddenly experience a drastic increase in them or notice some dark clouding, flashing lights, and/or partial vision loss. In that case a visit to an ophthalmologist is warranted to check for possible retinal detachment, which can compromise your sight if not taken care of quickly