To retard something is to slow it down. A lack of fresh fruit and vegetables can retard the growth of a young child.
This word should be used with a lot of caution. Calling someone who's mentally retarded — or of low intelligence — a retard is considered very offensive by many people. That use should be avoided. On the other hand, this is a harmless word when used as a verb. While skiing, a heavy, bulky jacket could retard your progress down the mountain. Keeping a plant out of sunlight would retard its growth. When you see retard, think slow and "Use with caution."