Use this phrase to talk about two different things that are both true. The first one is obvious or expected, and the second one is more unexpected. For example:

She's not only beautiful, but also strong and intelligent.

The second part of the sentence includes "but also" or "but __ as well":

Kids that age not only pick up everything they see, but they often like to stick objects in their mouths as well.

You don't always have to include "but also" or "but ___ as well":

I not only warned him; I absolutely forbade him to go.