"Supposed to" can be used to express facts or predictions that other people have made and you have heard. A weather forecast is a good example. The newscaster has predicted what the weather will be like. You have heard this prediction, so you repeat it to another person as "it's supposed to ___":

It's supposed to clear up by Friday.

Another example is renting a movie that's gotten good reviews:

This one's supposed to be good.

"Supposed to" also has other meanings in other situations, like when you're telling someone what they should be doing:

You're supposed to call if you're going to be late.