|The "motion induced blindness" optical illusion|
It's another example of how attention paid to one thing can cause other things to disappear.
The video creates this illusion using two types of backgrounds - one where the background blue dots all move in the same direction, and one where they move randomly - to see if background movement is a clue to the illusion.
If you're intrigued and want to explore more, here's an excellent page where you can tweak the various elements of the illusion, changing the dot sizes and colors and so on, at Michael Bach's site on illusions. In his interactive version, you stare at a center green dot and the outside yellow dots disappear. Visual Phenomena and Optical Illusions>>
Moving illusions: Now you see it, now you don't, New Scientist>>