“To hit the nail on the head” means to describe a problem or situation accurately.
In this context, the nail refers to the metal pin used in carpentry, not the nail on the tip of your finger.
The head is the flat top of the nail.
To drive a nail into a piece of wood, you have to hit the head at the correct angle.
Here are some examples of how to use the phrase.
Amy: Why are we losing money?
Jane: We spend more than we earn.
Amy: I think you hit the nail on the head.
Sue: Why is the store losing customers?
Mary: Our customer service is bad.
Sue: You really hit the nail on the head.