Something that affects the entire community — whether it's your town, your country, or the world — is public. Climate change, for example, is a public concern, as is finding a clean public bathroom.
You can use the word public as a noun, when you're talking about a large group of people. You might say that the public is getting tired of the inefficient bus system in your city, or that the new library branch is open to the public. The adjective public is good for discussing things that concern everyone. The Latin root word, publicus, means "of the people, of the state, common, or ordinary."