What Does Marx Mean by Alienated Labor?

In his famous Paris Manuscripts of 1844, Marx argues that a society organized around the principle of private property and the commercial production of commodities forces man to stand in opposition to his own nature in order to subsist, and that this self-oppositional stance is best described as ‘alienated’ (or ‘estranged’) labor. To fully understand what Marx means by ‘alienated labor’, and under what circumstances labor becomes alienated, we must therefore first understand what Marx means by ‘human nature’.