Without an AO the target and cross hairs will become blurred or go completely away (if fine) as the magnification increases. That blurriness or "out of focus" is called parallax.
With the adjustable objective part of the scope you can "focus" or adjust the "parallax".
Lower power scopes such as 3x9 have parallax adjustment via the eye piece. get above 10-12 power and there is more adjustment needed to keep everything in focus.
You may have seen scopes advertised with a "side focus" knob opposite the horizontal adjustment turret. This does the same as an Adjustable Objective.
