Saccades are fast refocusing actions of the eye between fixations. This enables a person to quickly look at a point in space without actively looking at anything else in between. The eye simply jerks into the new position.
Even when looking at a single spot, the eye drifts around. This process is called microsaccades. It ensures that individual photosensitive cells are continually stimulated in different degrees. Without this changing input, these cells would otherwise stop generating an output.