The formula for density is D=m/v (Density equals mass over volume)
This means that density is equal to the amount of matter "stuff" in an object in a certain amount of space, or volume. In SI units, density can be measured in kilograms per cubic meter , or grams per cubic centimeter.
Recall that we see density written like these examples:
13.534 grams per cubic centimeter (density of metallic liquid mercury)
1 pound per cubic foot (density of a "common" form of styrofoam)
Note that the volume measurements are in units, or in "just one" of the specified volume measures. We reduce the volume to get a "one" or to get that single unit. For instance, if we had styrofoam, and we measured 2 pounds of it and it had the volume of 2 cubic feet, we'd reduce that by dividing by 2 so we could get a single unit of the specified volume in our statement of density.
A last example might be that if we had 45 grams per 3 cubic centimeters, we'd have 15 grams per cubic centimeters after the division to get that single unit volume.