Marx's idea of capital is based on David Ricardo's idea of rent on land. Rent is possible because titled possession to a scarce piece of property allows the owners to extract money from tenants. Because businesses own the machines and the facilities that workers use, they have an unequal amount of power and use it to extract the equivalent of rent from workers. It's value they take just because they can, over and above the market level wage.
This initial advantage of businesses over workers allows them to accumulate large amounts of capital over time, intensifying the differences in power. The collection of corporations and businesses forms the capitalist system, where possession of capital, jobs on the side of capital, and business ownership leads to the creation of an upper class.
Correspondingly, the of people who as individuals find themselves at the mercy of capital form the working class.
I'm not an economist, by the way.