Interfacing, also known as stabilizer, is a light piece of fabric that is sewn or fused onto fabric pieces that need an added weight.  You would commonly use interfacing for collars, cuffs or on the facing of a shirt or jacket.  Interfacing comes in many different weights so you will find one appropriate for your fabric whether that be for knits, sheerweight, or heavier fabrics.  With the advent of fusible interfacing it has never been easier to incorporate interfacing into your everyday projects.