A side is a physical position to the left or the right of an object or person. It’s also another kind of position—like if you take your brother’s side in an argument, you say he’s right. Even if he’s not.
There are physical sides, such as when Clark Kent parts his hair on the right side and Superman parts his on the left. There are also figurative sides, which you could also call positions or opinions. When your friends take your side in an argument, they agree with you, and when you have to debate both sides of an issue, it can help you understand another person's perspective.