Header & Footer code, and Use Global
Both settings live in the Page Manager, alongside the other page properties.
Header & Footer
The Head is simply the <head></head> section of your page. Here you can insert links to external JavaScripts, CSS and more.
The Footer gives you access to the area before the closing </body> tag. You can insert JavaScript and other items there as well.
Use Global
If you want pages to share the same properties, use the Use Global checkbox. This applies the settings to all the pages.
For Keywords, Description, Head and Footer, if global items are specified a box appears below them, just in case you’d also like to configure page-specific content on top of the global values.