The document discusses the hydrological cycle, which is the continuous movement of water on, above, and below the surface of the Earth. It is driven by energy from the sun and involves evaporation of water from oceans, lakes, and soil into the atmosphere; condensation of water vapor into clouds; and precipitation of water back to the Earth's surface as rain, snow, or hail. This cycle is crucial for sustaining life as it regulates and redistributes fresh water on Earth.