What is called atmospheric refraction? Which phenomenon results from it?

The density of earth’s atmosphere is not same everywhere, the layer near the earth’s surface has more density than the air at higher altitude. So, the refractive index keeps on changing continuously and randomly. Hence, the apparent position of the objects in distant outer space also keeps on changing. This phenomenon is known as atmospheric refraction.

The phenomenon such as twinkling of stars, early sunrise and delayed sunset occurs due to this effect.