Density is the mass per unit volume. In other words, density is how heavy something according to it's size. So if something is very heavy and small in size, density is high. Mathematically, formulated as Density = Mass /Volume.

Si Unit of density is kilogram per cubic meters. A cubic meter of water weighs 1000 kilogrammes, so it's density is 1000 kilogrammes per cubic metre. The density of air (at sea level) is 1.225 Kg/m3.