From phpBB Development Wiki
Page header is needed to set the title tag in the overall_header.html. I think, that you have recognized, that the title in the html-file, which you have created, doesn´t stand in the browser. And you also recognized that there aren´t any html/head/body-tags. So, how you get your title in the browser? Therefor is a function, which named "page_header". This function needs a string as input. So you have to insert in this function your site-title as well. I hope you understand that.