What do defocus and internal focus mean in photography?

Out of focus: During autofocus, the camera indicates that the camera is in focus, but the actual focus of shooting is before or after the expected focus.

Internal focusing: the front lens of the lens does not move when focusing; This facilitates the use of filters such as polarizers and gray gradient lenses.

External focusing: when focusing, the front lens group stretches, shortens and rotates; This is not good for using filters such as polarizers and gradient lenses.