Economic subjectivism is the theory that value is a feature of the valuer and not of the thing being valued. That is, things do not have inherent value, but have value only insofar as people desire them.
Subjective value theory was developed in the late 19th century as an attempt to overcome the shortcomings of classical economics. The most influential value theorists were members of the Austrian School of economics such as Carl Menger and Eugen von Boehm-Bawerk.
Those who endorse subjective value theory (including virtually all modern economists) believe it is a refutation of intrincisist value theories, such as the labor theory of value, which is a cornerstone of Marxism.
The theory can be illustrated as follows: Imagine you are very hungry before dinner and see a piece of candy sitting on the kitchen counter. The candy seems very desirable to you, but you forbear on the candy, and wait for dinner. Dinner is very good and you end up stuffing yourself on three helpings. Afterwards, you again see the candy sitting on the counter but now you have no desire whatsoever to eat it because you are so full. The candy went from being of value, to being of no value, even though the candy has not changed. But you have changed, illustrating that the value of the candy is a property of you, not of the candy. If one accepts absolute intrincisim, then the change in the value of an unchanging piece of candy represents a paradox. If, on the other hand, a thing's intrinsic attributes influence but do not determine its value, the changes in value according to changing circumstances present no surprise to the observer.