What would you do if you were not afraid?”

“What would you do if” is a way to ask about a possible scenario which may or may not ever happen.

For example, “What would you do if your boss fired you?” is another question that helps you think about possibility.

The quote implies (suggests) that fear often stops us from doing things we want to do. We become afraid of failure or a negative outcome. The question by Sandberg is asked to make us think about what it would be like to not have any fear—and what we might accomplish!