ATOS is a common rating system used in American schools to find the reading level of books. It is very simple to understand; the ATOS number is equivalent to the expected grade level of the reader. An ATOS level of 3.0 means that a kid just starting third grade should be able to read it without too much difficulty. An ATOS of 3.5 means that a kid halfway through third grade should be able to read it. Despite appearing quite exact, these are rough indicators, and it is not uncommon for kids to read, and enjoy, books a grade level or three above or below their official ATOS level.
ATOS is associated with the Renaissance Learning program (AKA Renaissance School Excellence and Renaissance Place), and Accelerated Reader (AR). ATOS is essentially a replacement for the Lexile readability level system, which is not part of the Renaissance Learning program. ATOS has the advantage that it is open and free to use, and was normed using a rather large sample of American school students reading the actual books that it references ("more than 30,000 students, reading almost 1,000,000 books"), rather than a sampling of test passages. The ATOS formula does leave something to be desired, basically being an index of average sentence length, average word length, word difficulty level, and total number of words in a book. This is actually pretty good as far as such measures go, but it ignores idiomatic phrases, difficulty of concepts expressed, and other common stumbling blocks.
ATOS stands for Advantage/TASA Open Standard; no one ever ever ever calls it that.
Find the ATOS level of your book here.
Read more about Renaissance Learning here.
Read more about ATOS here.