One of the ways to use the phrase "supposed to ___" is to talk about what a person intended, or what they were trying to communicate. 

For example, if you see a number written down, but it's not clear, you can ask:

Is this supposed to be an 8?

Or if your friend says something that might be critical of you, you can angrily ask:

What's that supposed to mean?

This means something like "Why did you say that?"