Answer: A mirage is an optical illusion in which a distant object appears along with its inverted image below it, as if reflected from a sheet of water, on a hot road or in a desert. It is seen when the ground is very hot, so the air near the ground is hotter (rarer) than the air above it.
Meaning: on a hot day the air near the road or desert surface is hot and rarer, and the air above it is cooler and denser, so the refractive index of air increases with height. Light rays from a distant object (such as a tree) travelling downwards pass from denser to rarer layers and go on bending away from the normal (refraction) until they turn upwards (at grazing angles near the ground they finally get totally reflected). They reach the eye from below, so the light appears to come from the image of the object inside the ground, giving the illusion of water. This is a mirage.
Condition: a mirage is seen on hot sunny days over roads or deserts, when the layer of air near the ground is much hotter than the air above it.
One more example (textbook): objects seen beyond and above a Holi fire (or a hot furnace/stove) appear to be shaking. The air just above the fire is hot and rare and its refractive index keeps changing, so the direction of light rays from those objects keeps changing and they appear to move. (Twinkling of stars is another example of atmospheric refraction.)