Starch is a natural polymer made out of sugars.  When added to clothing, usually during ironing, it acts as a smoothing and anti-wrinkle agent that assists the ironing process.

Today’s laundry starches are usually aerosol products, and actually are formulated in several closely related categories, including ironing aids, fabric finishes and sizings.  The primary difference is in the amounts of starch solids and silicone ironing assistant contained in the product.  The products consist of starch, surfactants, silicones, and a small amount of propellant, in a water base.  Bluing, optical enhancers and perfumes may also be added.  All of these products help to make the clothing we wear attractive and smelling fresh.