The word just has several meanings that come up all the time:

1. Fair and right. When something is just, it treats people fairly according to what they deserve. A just punishment fits the wrongdoing: a student who whispers once might get a warning, while one who disrupts class repeatedly might lose recess. A just society tries to give everyone fair treatment under the law. The opposite of just is unjust, meaning unfair or wrong.
2. Very recently or barely. When you just finished your homework, you completed it moments ago. If you just made it to class before the bell, you arrived with no time to spare. “I just saw that movie yesterday” means it happened in the very recent past.
3. Only or simply. “It's just a game” means it's nothing more than a game. “I just want one cookie” means you want only one, nothing extra. This usage emphasizes that something is smaller, simpler, or less important than it might seem.
4. Exactly or precisely. “That's just what I needed” means it's exactly right. “It costs just five dollars” means precisely that amount, no more and no less.

The word shifts meaning based on context, but you can usually tell which meaning fits by paying attention to the words around it.