Symbolism, Imagery, Allegory
Surplus-value basically refers to the extra money capitalists make when they engage in the capital process.
Let's trot out some Karl: "The complete form of this process is therefore M-C-M', where M' = M + ΔM, i.e. the original sum advanced plus an increment. This increment or excess over the original value I call 'surplus-value'" (4.15).
Don't look at us like that. Karl did it.
Anyway, the idea is that capitalists start off with money, buy commodities with it (the C in the middle of that equation thingy), and sell those commodities to make more money, the M'—a.k.a. M-prime. The ΔM, or Delta-M—delta is just a symbol meaning change—is the result of M' minus M. That's the amount of gain or profit. It's—ta-da—the surplus-value.
Technically, surplus-value and profit aren't the same thing. Profit is a type of surplus-value that results from, well, profiting, like buying a company for five million dollars and selling it for six million. Your profit is a million buckaroos. But capitalists can also obtain surplus-value by lowering wages, or from interest (which is just M-M'), or through other means.
It's worth noting that Marx points out that capitalists have to seek surplus-value, which motivates them to exploit workers. Because if they don't seek surplus-value as much as possible, their competitors will do it instead, and that means that they'll put the lazy or nice or non-exploitative capitalists out of business.