The hip (also referred to as a hep or haw) is the fruit of a rose, and, like all fruit, it develops after the fertilization of the flower occurs. When the flower is spared from deadheading, many small seeds—called achenes by botanists—grow inside, and the hip swells. As the season progresses and these seeds mature, a fleshy layer called the pericarp develops around them. The pericarp is high in vitamin C and natural sugars, which is why rose hips are used both medicinally and in culinary creations such as teas and jams.

Source: Brooklyn Botanic Garden