What is Mylar?

Mylar is actually the trademarked name for a particular brand of polyester film from DuPont. It has been adapted for a variety of uses.

What does a design look like without the Mylar?

Because the rows of stitching are farther apart, your background fabric will show through more than a regular design. On the other hand, there are far fewer stitches in a Mylar design so it will sew out much more quickly than a regular filled design.