ADD stands for Attention Deficit Disorder. It is also called ADHD for Attention Deficit Hyperactivity Disorder. They are the same thing, though ADHD is the official name which doctors use.
ADD is something that kids, teens, or adults can have.
Some very famous people had ADD, for example Thomas Edison who invented the light bulb, the record player, and the movie camera. He was the greatest inventor of all time. Many people with ADD have a hard time paying attention to just one thing, or keeping their mind on the most important thing. This is called being distracted. It's like your mind jumps from one thing to another and you lose track of what you were doing.