The “Gorilla video” is an example of inattention blindness or change blindness. The idea is that people often miss large changes in their visual fields.

Christopher Chabris and Daniel Simons (2010) describe in the book "The Invisible Gorilla" additional research they did with eye-tracking equipment. Eye tracking is a technology that can track where someone is looking. It tracks where the foveal or central gaze is. It doesn’t track peripheral vision. Eye-tracking research done while people are watching the basketball/gorilla video shows that everyone watching the video “sees” the gorilla in the video, meaning that their eyes are looking at the gorilla, but only 50 percent are aware that they have seen the gorilla.
If you are paying attention to one thing, and you don’t expect changes to appear, then you can easily miss changes that do occur.