Symbolism, Imagery, Allegory
Okay, we're not gonna lie: this concept is a little confusing.
So, Marx has this labor theory of value. He says that the value of a commodity is the socially necessary labor time required to produce it.
Okay, right. But how does Marx come to this conclusion? Well, he looks at the fact that commodities are exchanged (bought and sold), and he argues that because exchanges happen, something similar must underpin different commodities—something must be comparable between one commodity and the other that allows them to be traded at all. Marx concludes that the commonality is abstract labor time. It's abstract in that it ignores (or abstracts away from) the differences in the specific types of labor that go into commodities; this theory just looks at the time involved.
Here's what Mister Marx himself has to say: "If then we disregard the use-value of commodities, only one property remains, that of being products of labour" (1.1.11). And how about this? "A use-value, or useful article, therefore, has value only because abstract human labour is objectified or materialized in it. How, then, is the magnitude of this value to be measured? By means of the quantity of the 'value-forming substance', the labour, contained in the article. This quantity is measured by its duration, and the labour-time is itself measured on the particular scale of hours, days etc." (1.1.14).
That's not all. Marx also goes on a little tangent about Aristotle (1.3.36-40): basically, old Karl here says that the Greek philosopher figured out that there must be something in common between different values for them to be exchanged—but he himself couldn't figure out what it was. That's because value is abstract human labor. In ancient Greek society, slave labor was considered dramatically different from non-slave labor. That's why Aristotle couldn't form an abstract concept of labor that equalized the differences. At least according to Marx.
The socially necessary bit in socially necessary labor time more or less means that if we're talking about the value of a donut (with sprinkles, anyone?), we're talking about society's average donut made by society's average baker—not some dude who's taking three hours to make a donut because he's too busy watching Maury on the TV in the corner.