According to the Larousse dictionary online, the capital gain is the increase in the value of a thing for causes that are extrinsic to him. The concept was developed by the German Karl Marx (1818-1883).
Marx considered that the added value is any value that the employee created above the value of its work force. This value, which can be defined as the work without pay to the worker, is appropriated by the capitalist. In other words, the added value is the basis of capitalist accumulation.
Understand the concept of added value, to take into account that each product contains a value that corresponds to the time of socially necessary labour for its production. The labour force is also considered by Marxism as a commodity whose value is linked to the necessary so that the worker arrives to survive and reproduce.
For example: If the worker has to work four hours a day to satisfy its basic needs and those of his family, and that his work day is nine hours daily, is that the capitalist is trying to take ownership of the added value generated in five hours. This value is new and additional (i.e. value), because it was part of any other component of the production process.
This appropriation of surplus value is the operation of capitalism. For Marx, the capitalist can increase the level of exploitation through the absolute surplus value maximization (extending the working day) or the relative added value (in deciding the value of the labour force).