According to Karl Marx in Das Kapital, necessary labour is the amount of time a person works for himself and provides for himself. In an eight hour workday if it only takes four hours of work to create a certain amount of commodities which will in turn be enough to pay a person's wage when the said commodities are exchanged for money then it is said that necessary labour is only four hours for that labourer. The extra or surplus created by the further four hours of work is the surplus value and it is this exactly that the capitalist makes profit from. Remember, selling at a huge markup or overpricing does not create profit, only surplus value does.