Don't Be A Dale is a saying that people from the 814 (Pennsylvania Area Code) use to give advise. Dale being a kid from the area that not a lot of people like, or give respect to. So using this saying is basically telling someone not to act, or say something Dale would say.
Kid 1: "Dude, I just landed a frontside shuv-it!"
Kid 2: "Dude, Don't Be A Dale!"