Stencils are patterns that can be used to mark a quilt top for quilting. There are many types on the market or you can make your own. Most are made from plastic as this lasts longer than cardboard.
When choosing a stencil you need to think about the size of your quilt. For example, I bought the purple stencil for a quilt border but it was much too wide. I had to settle with the blue stencil on the right as it fit the border width.