All class societies are based on the separation of the producers from the means of production. Under capitalism the means of production and distribution monopolised by a minority function as “capital”, as wealth used to produce other wealth with a view to profit.
The source of this profit is the unpaid labour of the working class. Being excluded from the ownership and control of the means of production, the working class can only get a living by selling their ability to work, mental and physical, to a capitalist employer for a wage or salary. But this wage or salary, representing the value of the labour power they have sold, is less than the value of what they produce. The difference is surplus value and belongs to the capitalists who have bought the labour power. It is the source of their profits and of all other property and privilege incomes.