The term autofocus means ability of the image sensor to focus an object automatically. It is in contrast to manual focus where an object is to be focused manually. When someone hold the camera in front of an object the camera image sensor sees the object, then to take a clear image of the object camera lens need to focus properly. When this process done automatically then it is called autofocus. An image without focus will be hazy. An image properly focused will be clear and vivid.
PDAF or Phase detection Autofocus is a method of achieving focus of an object. In PDAF method sensor make images from the light coming from the object and compare the images.