Interfacing adds structure to your sewing project (for example, the collar of a blouse or the waistband of fitted skirt). The pattern will tell you how much interfacing to buy. There are a few types of interfacing (woven, non-woven, sew-in, fusible…) and you can buy it in black or white. When in doubt, ask the fabric store staff what type of interfacing will work best with the fabric you’ve chosen. This is generally a good strategy when you’re starting out.