A Trillion cubic feet (1 Tcf = 1,000,000,000,000 cubic feet) is a volume measurement used by the oil and gas industry. A Tcf of natural gas is approximately equal to a quad of energy. The natural gas's volume is measured at 21ºC and 1 atm. of pressure. When transported, the natural gas is compressed. Increasing the pressure, decreases the volume, see the ideal gas law for more information on how compressible gases are.
One trillion cubic feet corresponds to a tremendous volume, a volume that is out of the grasp of human imagination. To help visualize this amount follow this link, which is a visual representation of the number 1 million - now times that by itself to get a trillion (a trillion is a million millions)! Although this volume seems unimaginably large. In the year 2016, the USA alone used 27.5 Tcf of natural gas.