An interfacing is made from either a non-woven, a woven or a knit fabric. This used to partially stabilize a garment and give it some extra body. When you look at a blouse for example, the collar usually looks somewhat "starched", except an interfacing is not done with a starch but done before a garment gets sewn together and can be iron-on or sewn in. The iron-on is easier to use, it has little dots of a glue-like substance that dissolves when put face-down on the backside of a cut out pattern piece. Set your iron on low as it can easily shrivel up and leave a blob of glue on your iron (and maybe even ruin your pattern piece). Look on the pattern to see what is recommended to interface and always have some on hand. Usually it is good to have a little bit of iron interfacing for either a knit or a woven purpose. The knit interfacing has some "give" without the garment looking to "solid". Interfacing comes in a variety of weight but when first learning to use this, a medium weight works on most (double it up is not solid enough).
Interfacing is mostly used on waistbands, collars