Attention Deficit Disorder (ADD) is a psychological disorder which causes extremely low attention spans, especially in children.
It's first mentioned in "Gnomes" when Tweek's parents say that the cause of Tweek's jitteryness is ADD. It is later mentioned when Timmy and the rest of the fourth graders are diagnosed with ADD in "Timmy 2000" (although even normal fourth graders couldn't have payed attention to the tests they were given). Timmy was mis-diagnosed by some teachers from school and doctor for having ADD because he can only say his name due to his mental retardation. Everyone else is diagnosed with ADD because the doctor thinks they can't pay attention. They are all given prescriptions to get Ritalin (actual medication for ADD) and everyone overdoses on it and becomes calm and laid back (much like marijuana users). Doctors then have an antidote for Ritalin called Ritalout (a play on the fact that the opposite of 'in' is 'out', so Ritalout should negate the effects of Ritalin).