Symbolism, Imagery, Allegory
Let's have a definition from Marx, Shmoopers: "We mean by labour-power, or labour-capacity, the aggregate of those mental and physical capabilities existing in the physical form, the living personality, of a human being, capabilities which he sets in motion whenever he produces a use-value of any kind" (6.2).
("We mean by labour-power..." Um, who's we, comrade? It's just you writing the book... But, anyway…)
So, labor-power is a pretty simple concept, but you can rely on Marx to write something more longwinded than socially necessary. Labor-power just means your capacity or ability to work. When you work at a taco stand, you're selling the boss your labor-power: Yo, hi, I'm here, I can make tacos for you. That's you selling your labor-power. Pretty simple, right?
As Marx says, "In order to extract value out of the consumption of a commodity, our friend the money-owner must be lucky enough to find within the sphere of circulation, on the market, a commodity whose use-value possesses the peculiar property of being a source of value, whose actual consumption is therefore itself an objectification of labour, hence a creation of value. The possessor of money does find such a special commodity on the market: the capacity for labour, in other words labour-power" (6.1).
And as Marx also says, "The fact that half a day's labour is necessary to keep the worker alive during 24 hours does not in any way prevent him from working a whole day. Therefore the value of labour-power, and the value which that labour-power valorizes in the labour-process, are two entirely different magnitudes; and this difference was what the capitalist had in mind when he was purchasing the labour-power. [...] What was really decisive for him was the specific use-value which this commodity possesses of being a source not only of value, but of more value than it has itself" (7.2.22).
Karl's key idea is that while it might take only 10 hours of labor a week for society to pay the cost of a laborer (that's what Marx calls "necessary labor"), your boss could make you work for 20 hours a week. You, the worker, are paid less than you contribute. Thus, your boss gets 10 hours of free work ("surplus labor") that isn't necessary for humanity to keep going—and your boss can keep the surplus-value in his pocket and buy a Ferrari while you're still riding your rusty bicycle and crashing into potholes.
Exploitation of labor is the source of surplus-value.