Spots or floaters are small particles in the 'jelly' or liquid part of the eye (known as the vitreous) that become noticeable when they move in your line of sight.
They are more noticeable when you are looking at a bright or plain background, which is why people notice them more when looking at a blank surface or at the sky.
With time, 90% of us eventually develop floaters in our vision, so it's usually normal, and the brain eventually learns to ignore them in most cases. In cases where they are very distracting, surgery can be performed to replace the vitreous ‘jelly’ in the eye, and this surgery is called a vitrectomy, and your optometrist can refer you to an appropriate surgeon.
Sometimes, however, a significant increase in seeing floaters or flashes in your vision, or an unusual ‘curtain’ that comes down in your vision can be a sign of the retina becoming separated from the rest of the eye, known as a retinal detachment. A retinal detachment is a sight threatening emergency that needs immediate attention, so it is best if you notice flashes or floaters to get them checked out in an eye test.