This type of visual effect is known as a 'floater'. It is usually due to small clusters of cells breaking off the inside wall of the eye and floating in the gel which fills the eye. Unless there are many of them or they are accompanied by other visual phenomena such as lights or a 'curtain' across part of the vision, they are unlikely to be significant.

It is always worth mentioning them on the next visit to your optometrist. If floaters are numerous, of sudden onset or accompanied by other visual symptoms, get them checked out urgently by an optometrist.