Mass percent composition is exactly what it says, it is the mass percent of an element within a compound. This is the percent of the compound's total mass taken up by a particular element.
You can find the mass percent of a particular element by using the following formula:
Mass percent of element = (mass of elment in 1 mole of compound / mass of 1 mole of compound ) * 100%
It is useful in distinguishing various compounds.