From the AR website:
What is Accelerated Reader (AR)?
AR is a computer program that helps teachers manage and monitor children’s independent reading practice. Your child chooses a book at his/her own level and reads it at his own pace. When finished, your child takes a short quiz on the computer. (Passing the quiz is an indication that your child understood what was read.) AR gives both children and teachers feedback based on the quiz results, which the teacher then uses to help your child set goals and direct ongoing reading practice.
Children using AR choose their own books to read, rather than having one assigned to them. This makes reading a much more enjoyable experience as they can choose books that are interesting to them.
Teachers and librarians help your child choose books at an appropriate reading level that are challenging without being frustrating, ensuring that your child can pass the quiz and experience success.