The field "Window title" is the title of the browser's window for this page.
If it isn't set, it will look like : Page Title | Website Title
If it is set, it will look like : Window Title | Titre du site
"Page Title" is the title of, the page, "Website Title" is the title of the website, set in the Settings > Website Settings panel.
A little text describing the content of the page.
This will make your SEO a little better. Google likes that.
Exemple 1 : The page's title is "Introduction", the window title isn't set.
Exemple 2 : The page's title is "Introduction", the window title is different.