Capitalism is a system in which the worker generates value through their labor, value which covers their wages, cost of materials, and other businesses expenses. For a business to be successful, the value generated by the worker must exceed the business expenses. This excess value is known as profit, all of which is claimed by the employer, simply because they were responsible for the initial investment in the business. In short, the worker generates value with their labor, said value is claimed by the employer, returning only a fraction of this value as wages, keeping the majority as profits. Under this system, the worker labors only to make others rich, they only ever see a fraction of the value that they bring to the business.
Socialism calls for businesses to be owned by the workers who run them, sharing profits and controlling the business through democratic vote. This system eliminates the employer/employee relationship, there is no boss who serves only to leech off of the value generated by others.