What is fusible interfacing?

Fusible is basically a fabric with adhesive on it.  It is used as a backing for the purpose of stability, or for adding body to the fabric.  There are several different types of fusible fabrics and they each have a different purpose.  Lighter weight fabrics generally require lighter weight fusibles.  Some fusibles are woven, some knit, and some are basically made from paper (usually called Pellon). A woven fusible would be used for strength, a knit if you needed some stretch,  and a Pellon is just a cheaper option.  There is a common misconception that Pellons are used because they are lightweight but that is untrue.  Personally, the lightest and finest fusible I have ever used was a knit fusible made in France.

The most common use of fusibles in garment construction is when it is applied to the facing of the garment.  Again, this is to give the garment a bit more stability and shape, especially when the garment is on the hanger (aka hanger appeal).