Percent composition in chemistry typically refers to the percent each element is of the compound's total mass.
The basic equation = mass of element / mass of compound X 100%
For instance, if you had a 80.0 g sample of a compound that was 20.0 g element X and 60.0 g element y then the percent composition of each element would be:
Element X = 20.0 g X / 80.0 g total x 100% = .250 or 25.0 %
Element Y = 60.0 g Y / 80.0 g total x 100 % = .750 or 75.0 %
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