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