graded readers are books that are “easier reading”, written for a certain level of language proficiency. often, they use simpler language, or only have a certain number of words or characters in them. they’re great for when you want to read a book in the language you’re learning, but you can’t quite manage all kinds of literature yet… so you want something that matches your level. in chinese, graded readers are usually sorted by HSK level, or number of characters one has learned.
the chinese ones will sometimes have both the chinese characters and the pinyin transliteration in the page, so that it’s easier to read. the issue with this is that our western eyes immediately float to the latin characters we’re familiar with, making the exercise a bit pointless.
and this is where the magic grid comes in! :) you apply it over the text, and it hides the pinyin lines, so that you’re left just with the characters themselves. i didn’t know what it was for when i first noticed it tucked into the back cover, but it’s such a simple and elegant solution — i love it!