This is a ruler created by DeLoa Jones. It is a guide that is placed around the hopping foot to give support while outlining applique or other odd shapes. The bottom side of this ruler is rounded (beveled) to prevent the ruler from getting caught on the seam or applique edge.
To see this ruler demonstrated by DeLoa, you can go to www.youtube.com and search "Deloas appliguide". It is a great how-to video.