What is a code of conduct and why does the community have one?

A code of conduct is a short, public document that sets the behavioural ground rules for a community: be respectful, assume good faith, no harassment, no discrimination, and stay on topic. It exists to make the project welcoming to newcomers, who are exactly the people most easily driven away by a hostile chat or meetup. It is not censorship of opinions; you can disagree strongly about technical decisions and still follow it. The code of conduct typically explains how to report a problem and who handles reports. Enforcing it consistently, even when the person breaking it is popular or skilled, is what keeps a community healthy. Moderators and meetup hosts are usually the people who uphold it day to day.