"If then we leave out of consideration the use value of commodities, they have only one common property left, that of being products of labour. But even the product of labour itself has undergone a change in our hands. If we make abstraction from its use value, we make abstraction at the same time from the material elements and shapes that make the product a use value; we see in it no longer a table, a house, yarn, or any other useful thing. Its existence as a material thing is put out of sight. Neither can it any longer be regarded as the product of the labour of the joiner, the mason, the spinner, or of any other definite kind of productive labour. Along with the useful qualities of the products themselves, we put out of sight both the useful character of the various kinds of labour embodied in them, and the concrete forms of that labour; there is nothing left but what is common to them all; all are reduced to one and the same sort of labour, human labour in the abstract."
Abstract labor exists as an abstraction of concrete labor, labor towards creating value from a commodity. Concrete labor is the labor that creates use-value from a commodity, it creates a commodity's physical properties. Abstract value, however, is ABSTRACTED from concrete labor, it is the labor of the entire productive labor of a human society as a whole rather than labor for the purpose of creating value out of a commodity. We can use abstraction of the concrete labor used to create two commodities (Marx uses corn and iron) to be able to compare the use-value and exchange value of a commodity. Concrete labor is socially necessary labor, it is fundamentally required to create value in a commodity, whereas abstract labor contains a multitude of labor that may not directly correlate with the value of a commodity, because it contains the entirety of labor. Concrete labor creates use-value, value on the basis of the expenditure of labor and the physical properties of a commodity, whereas abstract labor creates through the totality of labor the exchange value of a commodity, through comparing the concrete labor put into any commodities, and the labor required to create them, compared with the total abstract labor.