Density is defined as mass per unit volume. It is basically a ratio depicting how much stuff is crammed into a space of a certain volume.

The typical units for density are g/mL. The density of water is approximately 1 g/mL. Things that are more dense than water (have a higher density value) will sink when placed in water while things that are less dense than water (have a lower density value) will float. Most metals are more dense than water (they sink) while most oils are less dense than water (they float).