Meaning:

Say exactly a right thing. Answer to a question in exactly the right way. Be exactly right about something.

If you say someone has hit the nail on the head, you mean he/she has answered to a question or described something in exactly the right way.

Example:

She hit the nail on the head when she proposed her plan at the last meeting. It's exactly the thing we need to do now!

A: I think education is the most important thing we need to work on if we want to make this city a better place.
B: You've hit the nail on the head! I completely agree with you!

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