If the attacker is using a Physical move, and is at +2 Attack while the defender is at +1 Defense, then a critical hit will ignore the Defense boost but not the Attack boost.
The same occurs if the attacker is at -1 Attack and the defender at -2 Defense, with the Attack drop being ignored but not the Defense drop.
Basically, the quote sums up this:
(Special) Attack Drops are ignored.
(Special) Attack Boosts aren't ignored.
(Special) Defense Drops aren't ignored.
(Special) Defense Boosts are ignored.