The document defines density as the ratio of an object's mass to its volume. It explains that density is a measure of how much "stuff" or matter is contained within a certain space, with a higher density meaning there is more mass concentrated in a smaller volume. Examples are provided to illustrate these concepts, such as a solid object having a higher density than a gas because its molecules are packed more closely together in the solid phase. The document also discusses how density remains constant even if an object of uniform composition is broken into smaller pieces, since the mass and volume are reduced proportionally.