Meaning: This is a common way to tell someone to behave in a respectful and considerate manner. It's like saying, "Please act in a way that shows respect for others."
Choice of Words: The word "be" means to exist or to have a certain characteristic. "Polite" means to be respectful and considerate of others. When you put them together, "be polite" means to act in a respectful and considerate manner.