I think it's first of all an ethical idea. To "exploit" others is to treat them as means rather than as ends. In the ethical framework in which Marx's political economy is set, this is unethical. It is so in a sense inadequately conveyed by the notions of unfair and unjust. Relations of exploitation are less than ideal. They are "bad" from the perspective of both exploiter and exploited. "Good" relations are relations of "mutual recognition," a defining characteristic of which is the treatment of individuals always as ends and never as means.
This idea is part of a philosophy of history which views "development" as a process of "bildung" through which consciousness develops to rationality. One aspect of rational consciousness is rational willing; a successful developmental process creates a will for relations of mutual recognition. Such willing explains the ideal distribution rule, an essential function of which is to provide individuals with the specific means they require to develop and exercise a capacity for such relations.
In the historical stages of this developmental process, willing is dominated by "passions" rather than by reason. A defining feature of "passions," in Hegel and Marx's usage of the term, is that willing dominated by them produces results shared in by the community at large.
Though irrational, the "passions" dominant in capitalism are, in this sense, progressive in relation to those dominant in preceding stages. This explains Marx's claim that there were progressive aspects to British imperialist rule of India.
All sorts of questions can be raised about this framework. For instance, though its view of development is consistent with psychoanalysis in significant ways, it's also inconsistent with it in significant other ways.
In most treatments of Marx's idea of exploitation, however, this underpinning framework is ignored.
Ted