What is drought?

A drought is extended reduction in water availability. Droughts may be caused by insufficient precipitation or high temperatures which increase evapotranspiration. Contributing factors which may increase drought severity include excessive consumptive water use, such as lawn watering.

According to the dictionary it means "a prolonged period of abnormally low rainfall, leading to a shortage of water."

Basically it's when there is very little rain or water and the amount of water in a place decreases.