Literally: to have a nail fixed in the head. But don’t worry, not in reality!
Avere un chiodo fisso in testa means to be fixated on something; to have a mania, something that keeps our mind concentrated just on it. It could be something like “Jane thinks always about her boyfriend…she’s fixated on him!”
This is a common Italian slang! Would you like to learn more and more of them? Take a look to our Italian language personalized courses.