Manners are the proper or polite way to behave in public. If you take the chewing gum out of your mouth and stick it behind your ear before a meal, you need to work on your manners.
You can also call manners "etiquette." It shows good manners when you hold doors open for other people, say "please" and "thank you," and refrain from burping in public. Manners vary between different cultures: you're displaying good manners if you slurp your noodles in Japan, though it's bad manners in the U.S. And in some Middle Eastern countries, burping after a meal is actually a sign of excellent manners.