When kids grow taller, their parents sometimes have to let out the hem in their pants so they'll be long enough. The hem is the very bottom, folded edge of a piece of clothing.
Most of your clothes have at least one hem in them — at the ends of your sleeves, the bottom of your skirt, or along the edge of your t-shirt. When a person sews a hem, whether he's making a dress or a pair of curtains, he folds a very small amount of fabric under two or three times and stitches along the edge in a straight line. In Old English, the word hem means "a border."