A capitalist society is based on the systemic exchange of commodities. In this system, the application of labour to materials is itself a commodity; so workers are paid (a "wage") "for their labour". Otherwise they would be unable to buy commodities, and the system would not be based on systemic exchange of commodities.
But there must be some mistake in that. If I sell a ton of apples, this operation cannot create value, and will only transfer value if it is a lopsided purchase. If the apples are sold at their value, neither buyier nor seller will gain anything. Of course, in real practice, purchases are often made above or below the value of commodities; but this cannot be the rule, or the systemic exchange of commodities won't endure. Which means, as a whole, as a system, those deviations from value must cancel each others.
But how does it happen, then, that a profit is made in the apple trade?
That's because the producers of apples do not, in a capitalist society, sell apples. Instead, they sell their ability to work - what is called "labour power". But "labour power" is a different commodity from apples, and it has a different value. So there is a quid pro quo here: the company producing apples buys one commodity - the labour power of its workers (and it buys it, in normal conditions, at its full value) - but the end result of the operation is that it has now a stock, not of labour power, but of apples. If the value of apples is bigger than the value of the labour power implied in their production, then you have a systemic transfer of value - from workers to employers - that does not violate the conditions for a sustained system of commodity production.
(As it is easy to realise, if the value of the labour power implied in the production of a commodity is not smaller than the value of that commodity, then the production of that given commodity is impossible under capitalist conditions, or, in other words, if it is produced, it must be produced as a non-commodity.)
Evidently, all this is possible because labour creates value; in labouring, human beings create more than they expend.
But that is what Marx and Marxists call "wage slavery": the systemic transfer of value, from workers to employers, that keeps workers' economic earnings always close to the socially acceptable subsistence minimum.