The Earth is a closed ecosystem. The water on it dates almost from the creation time of the planet. Without water, the Earth would be a dead planet.
Water in nature transformed continuously, travels and recycled. Molecules of water we drink today, may were used in prehistoric times, millions of years ago. This perpetual recycling called "Water Cycle" or "Hydrological Cycle".
Under the influence of solar heat, the water present in the Earth's surface is constantly evaporate. Water vapor generated condense into clouds. As they come in contact with the cold layers of the atmosphere, returning to earth in rain, snow or hail.
A part of this water revaporised immediately. The remaining concentrated in streams and rivers flowing into lakes and then to the sea, from where it evaporates again. Another part penetrates the ground, forming underground rivers, lakes and aquifers emerging again on or reach directly to the sea.
The hydrological cycle renews the stocks of freshwater on Earth. But the amount of water is limited and its distribution in space and time uneven. Water pollution further restrict the quantities of. We have not always available as much water as we want, whenever we want and in fine quality.