Here, we start at a certain point in the population and keep selecting elements at a regular, fixed interval. In statistical terminology, we essentially select every k-th element, also known as the sampling interval, in the population.
For instance, when you are checking for quality control or auditing, you extensively deploy systematic sampling. It is not feasible to test the quality of all the products or accounting entries. By deploying systematic sampling, you get a strong sample on which you can base your inferences about the population.