When learning materials are graded, they have been built in steps, so a beginner is not handed the same English as a strong student. Teachers talk about graded readers (books that use only easy words at first, then slowly let in more), graded exercises, and a graded course or syllabus. The picture is a ladder: you stand on one step until you can take the next. People also use graded more loosely for anything sorted by how hard or how advanced it is, not only school books. Though actually, the same spelling is just the ordinary past of grade, “to give a mark”: She graded the tests. That is a different job. This word here is the “made for levels” one.