Can someone explain recommendations to me?

So, as my guidance counselor recommended I do, I have already asked my teachers for letters of rec. They are both working on them (one has even finished already). I thought all they had to do was write a letter and upload that through Naviance (my school uses Naviance E-Docs) to the school.
But on the Common App, I just learned that there is a chart that has to be filled out too- how does this work? Could someone with more experience explain this to me?