A "nail" (nail) is a long, thin piece of metal that has a point on one end - it's sharp on one end - and you use it to hold together two pieces of wood together.

You usually use a hammer to hit the nail into the wood.

The "head" of the nail is the flat part on top that you hit with the hammer.

Literally, "to hit the nail on the head" means to move the hammer so that it hits the top of the head of the nail and pushes it into the wood.

But here, we're using this expression to mean that Chris is exactly correct, or is doing something exactly the way it should be done.

Hannah means that Chris's memo said exactly what it should have said.

It "hit the nail on the head" - it addressed or talked about the issues that it should have talked about and addressed.

In other words, Hannah liked the memo.

But, she says, she wants to play the devil's advocate for a minute.

A "devil's (devil's) advocate (advocate)" is a person who presents an unpopular or opposing opinion in order to make the discussion better.

The "devil" is, in Christian belief, the spirit or the force that causes evil or bad things in the world.

The "devil's advocate" would be the lawyer for the devil.

An "advocate" is someone who is in favor of a certain position; a lawyer is an example of an advocate.

So, the "devil's advocate" is someone who tries to present another view - an opposite view.