Just means "fair." When something is morally and ethically sound, it's just. If you are a just teacher, you won't give your student an F just because his mother is rude to you.
Just might remind you of the word justice. When we describe a person, a rule, or a war as just, we mean that whatever has been done has been done for good reasons, and is fair to all sides. Just can also mean "only" or "simply." If swimming scares you, try the kiddie pool — it's just 3 feet deep. If you startle someone, say, "don't worry, it's just me." And something that happened only moments ago just happened.