Knitting (also applies to crochet):

Blocking is one of the final steps in creating a garment, to give one's knitting an even, professional finish, smoothing out minor imperfections and adjusting the final shape to the desired dimensions. Flat knitting is blocked prior to seaming to avoid inadvertent biasing when garment pieces are sewn.

There are two basic steps to blocking: dampening and shaping. The item is dampened, shaped and allowed to dry completely. Dampening can be done either with cool water or with steam.

Although blocking smoothes out minor imperfections, it cannot work miracles with a poorly knitted item. Garments can be blocked a little larger, wider and shorter, or narrower and longer; they really cannot be blocked smaller.