Volume is an amount of three dimensional space. It's usually thought of as having height, width, and depth, and is typically measured in SI units of cubic meters (m3). When asking how much is used, or present, it's often easy to give this answer in terms of volume. When the substance is incompressible (stays the same density when under a large range of pressures), this makes a fair amount of sense. Measuring gasoline, oil, and water in terms of volume is straightforward.
Gases are also measured in and sold by volume, but this is trickier because gases change their volume depending on pressure. When measuring a gas, which is very compressible (its density changes quite a bit when put under pressure), a standard temperature and pressure must be used. Different organizations assume different temperatures and pressures, but these pressures are generally about 1 standard atmosphere and between 0°C and 25°C.
For example, natural gas is often measured by the number of cubic feet (or millions, billions, or even a trillion cubic feet), and different organizations use slightly different pressures and temperatures for this, but these differences are comparatively slight. The volume of 1 trillion cubic feet of natural gas is roughly equal to 1 quad of energy.