While sitting in class or doing homework a lot of thoughts can pop into your head, one after the other. Or sometimes you might daydream a lot. It may be harder for you to tune out other kids in the class when you should be working. Many kids with ADD have a hard time staying organized and keeping track of things. They forget or lose things like homework, books, or supplies.
Some kids just make a lot of careless mistakes on school work.
It's not because they don't care, but because many times they get distracted, or they get very impatient and rush through things. It just means learning how to do the right things that help you do well. Your teachers, parents, or a counselor can give you some good ideas on that. Doctors know that it is genetic and runs in families. Most people are born with it.