This feature is generally used when you want the page name to remain simple and user friendly, while giving the search engines a title optimized with words people might search for to find this page.
For example, Amazon uses the title <b>Kindle Fire</b> as the H1 (page name/title) while using <b>Kindle Fire - Full Color Kindle with 7" Multi-Touch Display, Wi-Fi</b> as the <Title> tags (SEO Page Title).
Do not fill with Keywords only, as the search engines will likely penalize you. If you haven't done any research on an optimized title for this page, you're probably better off leaving this field blank and letting the Page Name represent both the H1 and the SEO title.