ADD stands for Attention Deficit Disorder. It is characterized by having difficulty focusing on one specific task. Instead, the afflicted is paying attention to multiple activities as opposed to one.
(For more information, go to: nami.org or mayoclinic.com.)
Billy can't focus on the teacher's lecture, and is instead watching the clouds, listening to the girl next to him blow bubbles with her gum, and imagining the sky purple. This is because he has ADD.