Header overlap option is implemented to allow your header elements to be on the top of your website content. It is usually used for a transparent header on the home page if you have some full-screen slider or large image on your home page content. You can enable it in Global header settings, the option is called “Make it overlap”. Note that if you want to apply it to one particular page only, you will need to create two copies of your header. One will be used as a default header for all pages and another one, with overlap turned on, set particularly for the home page for example.