A cubic feet per second (cfs) is a rate of the flow, in streams and rivers, for example. It is equal to a volume of water one foot high and one foot wide flowing a distance of one foot in one second.
One "cfs" is equal to 7.48 gallons of water flowing each second. As an example, if your car's gas tank is 2 feet by 1 foot by 1 foot (2 cubic feet), then gas flowing at a rate of 1 cubic foot/second would fill the tank in two seconds.
This unit is a non-standard unit and should not be used outside the USA. For international documents you should use Cubic meter (m³) per second according to the international SI-System.