The phrasal verb supposed to means that there is something that you should do. Whether or not you do it does not matter, but for some reason it would be good or important for you to do it.
Any time you use it, you could replace the phrase with “should” or “could”. The idea is that you already have plans. They are likely something written into your calendar or on your to do list, and you plan to complete them because it would be better to do so than to wait.
In context, you usually let someone know what you or they are supposed to do because they are not doing it right now. When you are at work, for example, your boss could come and talk to you. They would explain what you are not doing correctly by saying what you are supposed to do, rather than what you are spending time on now.
Or, your friend could invite you to go somewhere, but you already have an event that you promised someone you would attend. If you use supposed to in this context, it sometimes means that you would rather go with your friend to what they invited you to. However, you already have plans for something.
For example, see the following:
- I am supposed to go to my friend’s birthday party tonight. Could we reschedule to tomorrow night?
- When you have some free time, you are supposed to check on the bulletin board to finish tasks that the whole department is working towards.
- I thought the bus was supposed to leave at exactly 9:15; why are we still at the bus station?
Note that for all of these sentences, you could replace the phrase “supposed to” with “should”. They describe things that you need to do, rather than things that you want to do or are doing.