Density describes how "packed in" the mass is within that volume. In other words, how compact is the stuff.
EX: A marshmallow and a golf ball have the same size, but different mass. The golf ball has higher density (has more mass) EX: A kilogram of steel and a kilogram of feathers have the same weight, but different density. The kilogram of steel has a higher density.
Gases and Liquids have densities too! That's why they sometimes form layers when put in the same container.