It is a way of saving the rainwater which would normally flow off a roof and down the drain, and using it as piped water to flush toilets and for the garden watering, yard washdown, vehicle and car washing, instead of treated drinking (potable) water.
There are many government projects going on to save water for the future either by building dams or building reserviours etc. Even we can save water for the future by limiting our uses of water and avoid wasting it.....