In essence, good manners mean you don't make others uncomfortable around you. It's not just eating quietly and neatly, it's paying attention to the person who is talking to you, not gossiping, laughing even if the joke isn't funny (as long as it is not crude). Good manners are a form of caring.
They are also a bridge between cultures and lifestyles. Knowing the protocol of the other culture is a form of good manners. Allowing others to have the spotlight is a form of good manners.
If, by good manners, you are simply referring to table manners, put your napkin in your lap, use your silverware from the outside in, tear your bread in bite size pieces before you butter it, one piece at a time, and don't gulp. Chew with your mouth shut and wait until it is empty to talk. Eat small pieces at once so you are not looking like a cow chewing cud.
It's just a matter of making sure others are not uncomfortable around you. That's really the whole thing.
Bad manners hold you back in life
There are a lot of people who think that manners and etiquette are about forcing people to behave in unnatural ways, or that it's a way of making some people feeling stupid (when they commit a faux pas).
But in truth, manners create a framework that allows us to communicate with fewer misunderstandings, and with clear expectations for what will happen. Good manners are simply knowing how to behave in a way that doesn't make the people around you uncomfortable. It's NOT about knowing how to use weird appetizer forks, or about telling other people how to behave. It's just about being at ease in a social situation, and not unwittingly embarrassing yourself.
It's a pain to have to teach children good manners. Remembering those "please" and "thank yous," chewing with your mouth closed, speaking in polite volumes, not interrupting, etc., are not easy skills to acquire. But if you equip your children with these, they will be able to...