‘To hit the nail on the head’ means to describe exactly what is causing a situation or problem.
Examples
“He hit the nail on the head when he said this company needs more HR support.”
Or
“You’ve found the problem, John! You hit the nail on the head!”
The Origins
This phrase, quite obviously, has its roots in carpentry, however, it is not clear as to when it started being used. The analogy is simple, while hammering in a nail, you would want to hit it on its head to get the desired result. Missing the head could mean causing damage to the surface or even injury to yourself.