Stabilizers are designed to support fabric under the stress of machine stitching. There are a variety of stabilizers to choose from. The four basic stabilizers you need are: cut-away mesh stabilizer, basic tear-away stabilizer, nonwoven wash-away stabilizer, and adhesive nonwoven wash-away stabilizer.
Toppings are an additional layer of stabilizer to keep textured fabrics from poking through stitching and prevent the stitches from sinking into the fabric. They also improve the registration of small designs and prevent base fabric color from showing through the embroidery stitches. You can even get special effects using Mylar as a topping.