Have you crossed any bare farmland during the daytime?
Have you seen the shimmering in air above the soil surface?
The below video showing the heat shimmering of a land during summer with the background of Kangaroos: 

Do you know why this is happening?
Soil contains water. Due to heat, water in the soil evaporates. Evaporated water vapour above the soil surface reflects the sunlight which looks like shimmering

Soil can retain water, and different types of soils retain different amounts of water. Moisture is the quantity of water present in the soil.  Even dry soil contains a small amount of moisture. Clayey soil can retain the highest amount of moisture.

Take a boiling tube. Put two spoons of a soil in it and Heat it using a flame. We can observe a small amount of water droplets on the sides of the tube. Presence of water droplets is due to the moisture in the soil evaporates and condenses on the tube's sides.
This experiment shows the presence of moisture in the soil.