When you're supposed to do something, you're expected or required to do something. In your example sentences:

_"You're supposed to stop when at a red light" carries with it a variety of meanings and implications: "You are expected to stop", "According to the rules of the road, you must stop", "The police may catch you if you do not stop"

_"You're supposed to unpack once you get there."_ means someone wants you to unpack once you arrive at your destination - it might be the person speaking (your parent?), the person on-site (the camp counselor?).

_"Supposed to"_ can also be used when you talk about general rules of good behaviour, health & safety, or common sense, for instance:

- _"In the old days, men were supposed to stand up when a woman wanted to leave the table."_
- _"You're supposed to wash your hands after you've used the toilet."_
- _"You're supposed to recycle as much plastic as possible."_