Difference between the overall value generated by labor power and the value of labor power itself, that is, the social cost of its reproduction in a quality and quantity similar to that used.
Conditions for the production of surplus value
For surplus value to exist, there must first be the possibility of converting money into capital, that is, labor power must be a commodity that is freely bought and sold on the market. For this, the seller must be able to meet the buyer as persons who exchange commodities under "equal rights".
The worker must be "free", that is, the owner of the commodity he is going to sell, and willing to sell his labor power for a definite time as something separate from himself -if he sold himself he would be a slave, and if his labor power did not belong to him by right he would be a serf. For all this, besides having no alternatives, he must be dispossessed of the means of production that would allow him to turn his labor power into commodities by himself.
For the conversion of money into capital, therefore, the owner of money must find the free worker in the commodity market; free in the double sense that on the one hand, as a free man, he disposes of his labor power as his own commodity, and that, on the other hand, he has no other commodities to sell; he is exempt and deprived, rid of all the things needed to set his labor power in motion.
Karl Marx. Capital, 1867.
As can be seen, capital and wage labor appear within a historically very precise, and in fact very recent, framework of social relations.
What characterizes the capitalist epoch, then, is that labor power takes for the worker himself the form of a commodity that belongs to him, and his labor the form of wage labor. On the other hand, it is only from this moment that the commodity form of the products of labor becomes generalized for the first time.
Karl Marx. Capital, 1867.