Marxism considers power resulting from material possession. To him, power results from the systems created by people; specifically for us, this means capitalism. In this system, happiness comes from the ability to purchase and own luxury items. Those who are able to produce more goods, therefore, are considered more valuable to society, and are rewarded with money. Money not only provides the ability to purchase more, but it also can be accumulated in order to procure the means of production, thus moving that individual into the upper classes and giving them power with their new-found wealth.
Nothing besides the individual's ability to produce is considered of any real worth. It all comes down to how much one can produce. Whether it's a tangible item in a factory, or providing a service for which people will pay well, one's worth is determined by what they can do to produce tangible profit.
As such, women are disadvantaged in this system. Traditionally, women are responsible for the tasks of maintaining the household, something which detracts from the ability to do whatever it is they would normally do to be "valuable" in capitalist society. Even in a capitalist, but otherwise progressive society, women are biologically responsible for pregnancy and childbirth, something absolutely necessary for any society to continue to effectively function, but, similar to maintaining the household, something to which no concrete value can be assigned, and so it goes unrewarded by society. Even if this were not the case , the fact that women earn lesser wages on average than men gives men an inherent power in the capitalist system according to Marx.