Normally, light waves from the sun travel straight through the atmosphere to your eye. But, light travels at different speeds through hot air and cold air.
Mirages happen when the ground is very hot and the air is cool. The hot ground warms a layer of air just above the ground.
When the light moves through the cold air and into the layer of hot air it is refracted (bent).
A layer of very warm air near the ground refracts the light from the sky nearly into a U-shaped bend. Our brain thinks the light has travelled in a straight line.
Our brain doesn't see the image as bent light from the sky. Instead, our brain thinks the light must have come from something on the ground.