Marxism
Here is an example of exploitation of labor.  Jane is a shoemaker. She works for Michael, who owns a shoe factory that can make 60 pairs of shoes in a day. Jane makes 60 pairs of shoes every day. Michael pays Jane $20 a day.  However, Michael sells each pair of shoes for $2 each.  This means he makes $120 in a day.  After he pays Jane her $20 wage, Michael has $100 left over. However he then has to pay for materials which cost $1 for each pair, so that's $60 each day. Then running expenses of the factory cost him $10 a day. So he only gets $30 at the end of the day for managing the business.  This remaining wealth is called "Profit" or "Surplus [extra] Value." In other words, even though Jane makes 60 shoes every day, she only gets paid the value of 10 pairs of shoes. The rest of the day, while she is making the other 50 shoes, she is creating money for her boss.  Her labor is making him richer and helping him earn money.