It’s a three dimensional measurement. If you think of your cock as a cylinder, volume is the measurement of how much space is inside the cylinder or in this case, your cock. As your length and/or width increases then the space inside your cock has also increased. This is typically expressed in cubic units. So the 0.48 inches is 0.48 cubic inches. If you want to know the volume of a box, it’s length times width times height. If you want to know the volume of a cylinder, such as your cock, then Pi and the radius come into the equation, but it’s all the same, it’s a measurement of how much space is inside a three dimensional object.
Hope that also helps a bit.