You can call an idea which a lot of people believe but isn't true a "myth".
There's this myth that if you work hard you'll be successful. The reality is that things don't always work that way.
There's this myth in our society that you have to have a career to be happy.
The word "myth" originally means stories about gods, goddesses, monsters, and heroes from long ago. But when you use it in this phrase, it means incorrect ideas that a lot of people believe.