This type of visual effect is known as a 'floater'.

It happens due to small clusters of cells breaking off the inside wall of the eye and floating in the gel that fills the eye.

If you've only got a few of them, they're unlikely to be significant but it's always worth mentioning them on your next visit to you optician.

If you're experiencing many of them or if they're accompanied by other visual phenomena such as lights or a 'curtain' across part of your vision, get them checked out urgently by an optician.