To "clarify" something that you said means to make it clear. When you've said something that might be confusing or was misunderstood, you "clarify" your statement by saying it again in a new way and adding more details. The word "clarify" can be used in questions like in the example above, or in the phrase "To clarify,..."

Just to clarify, you said that you're definitely coming, right?

You often hear the noun form of "clarify", which is "clarification":

I wrote back to her and asked for clarification, but she never returned my e-mail.