It's like a translation or the bible. For example - in some bibles on one side is the bible's original text while on the page beside it there's the side that's broken down and explains things in modern language - or well, easier to explain language. You'd do this if you're a school teacher, or your children want to start reading the bible. This way you can keep going swiftly and there won't be many questions asking "what's that mean ?"
I hope this helped, and you pass! (: Good luck, rockstar!