To hit the nail on the head means to describe exactly what is causing a situation or problem.

For example:

Tom hit the nail on the head when he said most people can use a computer without knowing how it works.

I think David hit the nail on the head when he said that kids don’t want to buy this product.

She hits the nail on the head when she correctly identifies the absolutely perfect accessories for her outfit.

Bob: Tom, you’re not seeing what’s right before you. Susan doesn’t want to continue working at our company, so she misses work and doesn’t do her job well. She wants us to let her go.”

Tom: You’re right, Tim.That explains a lot. I didn’t think she could be this unprofessional. I think you hit the nail on the head. What a disappointment!”

Tradução: acertar na mosca, acertar em cheio.