AF, in this context, refers to autofocus. Using the autofocus function of a camera, you’re allowing the focus motor to adjust to change focus instead of manually moving it yourself. When working in the autofocus setting, you can also often select the focusing mode as well.
Most cameras come with two prominent focusing modes: continuous-servo and single-servo. Continuous-servo is referred to on Canon cameras as Al-Servo and on Nikon cameras as AF-C and allows the camera to continuously adjust the focus as long as you’re pressing down on the focus button. This means that if you’re photographing a moving object, you can use the continuous-servo mode to engage in autofocus tracking.
This means that focus tracking is an autofocus function in which the camera is continuously analyzing a moving subject and predicting where it will move to maintain focus. Focus tracking is especially useful for action shots and wildlife photography.