this reality should be recognized as the work of our own hands. Not to see it as this--to regard it as something natural or inexplicable, independent of our own activity--is what Marx calls alienation. He means the condition in which we forget that history is our own production, and come to be mastered by it as by an alien force.
Labour as we know it is an alienated form of what he calls "praxis"--an ancient Greek word meaning the kind of free, self-realising activity by which we transform the world.
Marx writes wonderfully in the Economic and Philosophical Manuscripts of the protean, shape-changing, alchemical nature of money