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."