If you want to edit how the top of the page looks, or add any custom meta tags (for search engine optimization) this is the file you want to edit. There is some PHP in this file however most of it is HTML. This will be the header for every page so if you change the meta information you might want to keep it dynamic so it better adapts to the different pages of your blog.
Also remember that any changes you make to this file only apply to the theme who's folder you are working in. If you change themes, these changes will no longer be applied.
