“This one's supposed to be good.”
You're at a video store, looking for a DVD to rent for the weekend. You see a movie that you read a positive review of. You want to convince your wife to watch it, so you say this.
This one's supposed to be good.
"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.