Use this phrase to share an opinion that you've heard in a few different places. For example, if you heard something once from your cousin, read it once on a website, and so on, you can say:

Some people say you can tell whether a child is going to be a boy or a girl based on what kind of food the mother craves.

When you say "some people say", it seems like you're not sure whether it's true or not. You're waiting until you have more information.