It's for a paper piecing class, right?
Here's a picture:
http://www.buy.com/prod/add-a-quarte...217369647.html
It also comes in a 6 inch length, which might be easier to deal with depending on the blocks you're making.
It's a fabulous little notion to have, and I use it ALL the time when I'm paper piecing.
You can pp without it, but if your teacher is going to be using it in class for a new technique (to you), you're probably better off having it. I'm sure the shop sells it.