Percent composition tells you which types of atoms (elements) are present in a molecule and their levels. Percent composition can also tell you about the different elements present in an ionic compound as well.
Let's look at two examples
The molar mass of #H_2O# is 18 g/mol
The hydrogens make up 2g (since each mole of hydrogen is 1g)
The oxygen makes up 16g.
The percent composition of the compound is:
H = (2g/18g) x 100 = 11.1%
O = (16g/18g) x 100 = 88.9%
Percent composition can be calculated the chemical formula of a compound, or it can be determined experimentally.
Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound.
Video from: Noel Pauller