Control joints are grooves inserted into a concrete surface to "control" cracking. Essentially, this groove is an intentional, controlled crack inserted to preclude the concrete's cracking on its own, in an uncontrolled manner. By placing control joints in it before it cures, any stress the concrete will be subjected to subsequently will not produce haphazard cracks that will be a landscaping eyesore.
With a trowel or jointer, the mason can cut an even control joint that will be aesthetically pleasing. Control joints can also be cut into existing concrete surfaces using a saw with a masonry blade attached.