The Auto Focus (or Autofocus) is a camera setting that determines the subject of the focus of a camera. There are different kinds of Auto Focus you can use, single-shot or continuous, which are best for standing still and moving objects respectively. With Auto Focus enabled, slightly holding the shutter release will let the camera focus on an object. If auto focus is disabled, you have to manually adjust the focus until the object or part of the image you want to be sharp is put into focus.
A camera uses a number of sensors to automatically focus on an object or spot. The number of sensors may vary from camera to camera, however advanced cameras tend to have more focus points than less advanced ones.