Density is the mass of 1 ml  ( = 1 cm3) of a solid, liquid or gaseous substance.
 

1 ml water weighs 1.0 g,  so its density is 1.0 g/ml,
1 ml aluminium weighs  2.7 g,  so its density is 2.7 g/ml,
1 ml iron weighs  8.0 g,  so its density is 8.0 g/ml.

Light metals (aluminium) are metals with a density of  < 3 g/ml
Heavy metals (iron)      are metals with a density  of  > 3 g/ml