In Marxian economics, surplus value is the difference between the amount raised through a sale of a product and the amount it cost to the owner of that product to manufacture it: i.e. the amount raised through sale of the product minus the cost of the materials, plant and labour power.
This a Marxist idea that the worker is being exploited by the employer, because the employer does not pay him for the full value of his work. If he did, there would be no profit, and since no employer is going to hire someone without earning a profit, any situation where a capitalist hires a worker is exploitation.
Here's why I think this is wrong.
Consider two workers, Tim and Tom. They are identical twins. Not only are they genetically the same, but for this example they also have exactly the same skills, experience, education, etc.
They are both cabinetmakers. Tim runs a one-man operation building and installing cabinets for homeowners and contractors. Tom works at another cabinet shop for wages.
When Tim spends a day building a $500 cabinet, all of that $500 goes to him. How much he actually earns will be $500 less the material costs.
When Tom spends a day building a $500 cabinet, his boss gets the $500. The material cost is the same that Tim had to pay, but for the labor portion Tom will earn less per day than Tim, with Tom's boss keeping the difference.
Is Tom being exploited? No, because Tim and Tom are not doing the same job. Tim has to drum up work, because without work, he starves. Tim has to make sales calls, come up with estimates for prospective jobs, and deal with idiot customers who want everything for free. Tim is always under time pressure and always dealing with stress-inducing deadlines.
Meanwhile, Tom works his 8 hours, then goes home to have a beer and relax. Tom doesn't have to deal with all of the bullshit, pressure, and headaches Tim goes through because he has essentially outsourced all of it to his boss. That's his choice, and there is nothing exploitative about it. It's a matter of trade-offs, and just because somebody chooses to have an easier work-life for less money, doesn't mean they are being exploited.