Most animals contract and dilate their pupils depending on the light available around them. Geckos are no exception but unlike other species the difference between those two stages are much more dramatic. They go from having cute round pupils in low light to evil narrow slits when it's too bright.
Not that this isn't awesome, I love reptiles, but this kind of pupil isn't that special. Cats go from round to slit as well depending on light intensity. I've seen my cats in low light with perfectly round huge pupils and in bright sunlight with an almost un-noticeable narrow slit pupil.