Interfacing is a layer that is used to add firmness, structure and support to certain areas on the inside of garments. It is commonly used on collars, cuffs, facings and pockets. Different varieties are available such as fusible, sew-in and knit. It is also available in black or white so that you can match it to the fabric you are using. You can also buy different weights, light to heavy, which you choose based on the weight of your fabric.