[1][2] In this text, Marx sought to refute the theoretical basis for the economic policy of Ricardian socialist John Weston.
[2] In the process of criticizing Weston, Marx's explicates his theories of surplus value and the falling rate of profit in simple and concise English.
In other words, profit is what is left over after paying the worker a wage representing a certain portion of the labour performed, the remainder effectively being unpaid and reserved to the capitalist.
Because this arrangement depends ultimately on the social conditions of labour and production, despite the existence of apparently natural laws governing the value of commodities, within these limits workers can organize around demanding a higher rate of pay at the expense of the profits of the capitalist, not at their own expense as argued by Weston, who claims that capitalists will simply raise prices in order to sell the same quantity of produce at a rate that will pay for the same quantity of labour, effectively cancelling out any wage gains won by workers through union activities.
[3] Marx argues that profit is derived not by selling commodities above their value, in which case capitalists could raise prices at whim, but that commodities sold at or near their natural value produce profit because workers are only paid for that portion of their work which pays for their own labour power, i.e. that labour which generates enough value to pay workers their wages.