Interfacing is a textile

Textile

A textile or cloth is a flexible woven material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by spinning raw fibres of wool, flax, cotton, or other material to produce long strands...

 used on the unseen or "wrong" side of fabrics to make an area of a garment more rigid.

Interfacings can be used to:

• stiffen or add body to fabric, such as the interfacing used in shirt
  Shirt
  A shirt is a cloth garment for the upper body. Originally an undergarment worn exclusively by men, it has become, in American English, a catch-all term for almost any garment other than outerwear such as sweaters, coats, jackets, or undergarments such as bras, vests or base layers...
  
   collars
• strengthen a certain area of the fabric, for instance where buttonhole
  Buttonhole
  Buttonholes are holes in fabric which allow buttons to pass through, securing one piece of the fabric to another. The raw edges of a buttonhole are usually finished with stitching. This may be done either by hand or by a sewing machine. Some forms of button, such as a Mandarin button, use a loop...
  
  s will be sewn
• keep fabrics from stretching out of shape, particularly knit fabrics

Interfacings come in a variety of weights and stiffnesses to suit different purposes. Generally, the heavier weight a fabric is, the heavier weight an interfacing it will use. Most modern interfacings have heat-activated adhesive

Adhesive

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 on one side. They are affixed to a garment piece using heat and moderate pressure, from a hand iron for example. This type of interfacing is known as "fusible" interfacing. Non-fusible interfacings do not have adhesive and must be sewn

Sewing

Sewing is the craft of fastening or attaching objects using stitches made with a needle and thread. Sewing is one of the oldest of the textile arts, arising in the Paleolithic era...

by hand or machine.