A tuck is kind of like a reverse dart in which the fold of the fabric appears on the right side of your project and you're basically pinching two sides of the fabric together.  It is done for purely decorative purposes and it definitely gives your project a unique look.  You can easily just do a couple or create many tucks until you get your desired look.   For best results make sure to try it on sturdier fabrics such as cotton, twills, fleece, and denim.