First of all I resent the term. As far as I'm concerned most people have Attention Surplus Disorder. As far as I can see, many people who don't have ADD are charter members of the Congenitally Boring. But anyway, be that as it may, there is this syndrome called ADD or ADHD, depending on what book you read. Some people say the so-called syndrome doesn't even exist, but believe me, it does.
Many metaphors come to mind to describe it.
It's like driving in the rain with bad windshield wipers. Or it's like listening to a radio station with a lot of static and you have to strain to hear what's going on. Or, it's like trying to build a house of cards in a dust storm. You have to build a structure to protect yourself from the wind before you can even start on the cards. In other ways it's like being super-charged all the time.