The production of surplus value is directly related to the rate of exploitation of workers in the workplace (total surplus value divided by wages). There are essentially two ways to increase this rate.
The production of absolute surplus value entails an increase in the amount of total value produced, usually by increasing the workday of workers, but also by intensifying the work done, by limiting breaks, supervision by management, and so on. This form of increase in surplus value is limited in its usefulness, since there are natural limits to it, such as the 24 hours of the day, but also the social limits, such as the moral welfare of the working population.
The production of relative surplus value, however, doesn’t suffer from these limitations, making it the main way of increasing surplus value for the capitalist. Relative surplus value is produced through the reduction of the value of labor power (variable capital) by means of improvements in the production of goods (effectively the appropriation of productivity gains by the capitalist class). In this case, with the working day and wage remaining the same, the value of labor power falls leaving a higher surplus value. There are several ways to achieve this result, such as introduction of better machinery, a better organization of the workplace, and so on.