A chew toy is a small rubber or plastic toy for dogs (or other pets.) Dogs love chew toys because they can bite them and viciously shake them around. Some people use old stuffed animals for chew toys, but there's a risk that it can break apart and the dog might accidentally swallow stuffing or something and get sick
Human babies get chew toys too, but they're usually called "teething rings" or "teething toys". Babies use them to alleviate the pain from teething.