Whereas in single-servo autofocus, the camera will only engage autofocus once each time you press the focusing button.
Your camera likely has the option of AF-S (single-servo autofocus) or AF-C (continuous-servo autofocus). When shooting with continuous autofocus, your camera will keep refocusing constantly so long as you hold down your focusing button. Whereas in single-servo autofocus, the camera will only engage autofocus once each time you press the focusing button.