Your stance is your posture or the way you stand. Figuratively, if you take a stance against bullying, you are standing against it.
If you take a stance on a contentious issue, it means you believe strongly about it one way or the other. If your stance is unpopular, you'll need some courage to speak your mind. The word comes from the Italian stanza which means stopping place. Your stance is something that's not likely to change. You have stopped there, your decision is made. You're done.