"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
Production that has high intensity of labor requires more hours of human effort and fewer tools and automation. As better tools and automation become readily available, intensity of labor drops.