“to tell the future” means to predict (or foresee) an event that hasn’t happened yet. This expression has the word “tell” in it, but actually the meaning is not related to “saying” anything. It is related to “reading” or “seeing” the future.
Geoff: Emma! I’m so scared! What are we going to do?!?
Emma: Calm down, Geoff! What are you talking about?
Geoff: I just learned that the ancient Mayans could tell the future! 2012 is the last year on the calendars they made, so the end of the world is coming soon! They knew about it over 1,000 years ago!!!
Emma: Yeah, I heard about that. I’m not worried about it, though. Even if it is true, I think you should try to chill out and enjoy life. Don’t freak out!
Geoff: OK, Emma. Maybe you’re right. I’ll try to relax.