Density is a physical property of every object that describes the mass per volume ratio. Using information about the mass (amount of stuff) and the volume (the amount of space) for two or more objects, one can make comparisions of their relatice densities.
 

The density of a solid is the mass-to-volume ratio. In this question you have to rank the density of three solid samples. The volume is determined by water displacement. The method goes like this: start with a known volume of water in a graduated cylinder (100.0 mL). Then add the solid to the water and observe the amount by which the volume reading increases. The amount that the volume increases by is equal to the volume of the solid. You can inspect the new volume reading for the three samples and notice that they all increased by the same amount - by 100.0 mL to the final value of 200.0 mL. And so it is the mass of the sample that matters here. The mass reading is in units of gram (abbreviated g) and can be read from the mass balance. The sample with the greatest mass is the one with the greatest density. Similarly, the sample with the smallest mass is the one with the smallest density.