We can define labor as human activity that produces a commodity*. Labor power is the ability to perform labor, very much like potential energy in classical physics. We make the distinction because under capitalism, the ordinary working person exchanges his or her labor power for some price; the purchaser of the labor power then uses the labor that the labor power affords to create a commodity.
*Technically, labor produces any item of value. Since most items of value under commodity relations are intended for exchange, I've cut out the terminological middle-man for the sake of brevity.
Labor power has a cost: the cost to feed, clothe, house, and maintain the person who embodies the labor power in a fit condition to perform labor, as well as the cost to reproduce, educate and train the next generation of laborers. Labor power has use-value: it can be used to create commodities that directly or indirectly have subjective value. Labor power is in fact exchanged for a price.
Thus: labor power is a true commodity.
Thus: the price of labor power under commodity relations will tend over time to its cost.
Thus: those people who exchange their labor for a price will, over time, receive the minimum necessary price for survival and reproduction.
Human beings have an amazing capacity for enduring hardship and misery as the price of survival; the cost of labor power — and thus its price — can thus be made depressingly low.
In my introduction, I enumerated some of the horrific and intolerable conditions of present day society. But just the bare fact of those conditions is not enough — and should not be enough — for a rational person justify a radical change abandoning capitalism. It might be the case that these problems are no more due to capitalism than the misery and suffering caused by earthquakes or volcanoes. It might be the case that we just have the "wrong" kind of people in the capitalist ruling class.
All of the above may be true, but even the nicest, most well-intentioned and intelligent person must submit to the larger social and economic relations of a society. It is inherent and ineluctable under commodity relations that as long as labor power is treated as a commodity, those who exchange their labor power will tend over time to receive the minimum necessary — which ain't a lot — for survival and reproduction, no matter how much surplus their actual labor produces.