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.
Nicely put! I have both the 6" and the 12" add-a-qtrs, and when paper piecing, my work comes out SOOOO much better, and my trimming is A-1. I went to a Carol Doak workshop without one and within 30 minutes bought the two I have. While other rulers/markers work well, I wanted to really hone my precision for foundation piecing. I wouldn't trade them or set them aside for anything else; although I don't FP all that often, when I do I know my work is precise.
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.