Value of labor power is the twin of surplus-value. It is the quantity of labor power required to produce the necessities of the worker. This includes but is not limited to, the need for housing, food, child-rearing, healthcare, clothing and so on. As society has increased the means of production further and further thru the use of better and more numerous machines and tools, productivity has increased, decreasing the amount of labor-hours required to provide the various needs of life. Somewhat counter-intuitively, as productivity goes up, the value of labor power goes down. This is due to the three laws that govern the inversely proportional relationship between the value labor power and surplus-value.
Laws of value of labor power and surplus value
Per chapter 17 of Capital vol. 1,
(1.) A working day of given length always creates the same amount of value, no matter how the productiveness of labour, and, with it, the mass of the product, and the price of each single commodity produced, may vary.
If the value created by a working-day of 12 hours be, say, six shillings, then, although the mass of the articles produced varies with the productiveness of labour, the only result is that the value represented by six shillings is spread over a greater or less number of articles.
(2.) Surplus-value and the value of labour-power vary in opposite directions. A variation in the productiveness of labour, its increase or diminution, causes a variation in the opposite direction in the value of labour-power, and in the same direction in surplus-value.
(3.) Increase or diminution in surplus-value is always consequent on, and never the cause of, the corresponding diminution or increase in the value of labour-power.
Socially necessary labor-time
"Socially necessary labour-time is the labour-time [literally the number of minutes, hours, and so on] required to produce any use-value under the conditions of production normal for a given society and with the average degree of skill and intensity of labour prevalent in that society. "
— Karl Marx, Capital Vol. 1
Under capitalist society, production of commodities depends on two things: the amount of capital invested in fixed costs, such as machines and in variable costs, which is only ever labor. When more of this investment for a commodity has to go to labor costs, rather than to fixed costs, the commodity's production is said to have high intensity of labor. As better tools and machines become readily available, the hours of human labor required to make that commodity drop, and so the intensity of labor for that commodity drops. This quality of production is the counterpart to the quantity that is socially necessary labor-time.