Should I tell what the function is or not?

Sometimes, when I design the layout of an app, I always think about that should I put a short description to tell user what this function is or how to use it. But, at the same time, I also think about that the user might learn everything after use this function one time, and maybe I don't need to put the short description. And, it will make the page cleaner and have more space.

I just wanna to know how to determine what should I do, when I design the user interface.

Should I tell users everything? Should I let users learn it by themselves?