Molar volume is another way to view number density. The molar volume is the volume of 1 mole of substance. It can be simply found by dividing the volume of a sample by the number of moles of that sample
\[V_m = {V\over n}\]
This is simply the inverse of the number density. It is a useful quantity to "think about" things from a molecular perspective. As all gases that are behaving ideally have the same number density, they will all have the same molar volume. At STP this will be 22.4 L. This is useful if you want to envision the distance between molecules in different samples. For instance if you have a sample of liquid water, it has a mass density of 1 g mL-1. Since water has a molecular weigh of 18 g mol-1, 1 mole will have a mass of 18 g. From the density this should have a volume of only 18 mL. Clearly this is a lot less than an ideal gas at STP which is more than 1000 times larger in terms of its molar volume.
In the video given below, Dr. LaBrake talks about the Standard Molar Volume of an Ideal Gas.