I believe 'user agent' is the software used to display your page. In this case Qt as used by Sigil, but it could be Firefox, IE, or the software on your ereader device.
These settings can, I believe, be overridden if you define them in your stylesheet. You'll also notice they are there if you do not link any stylesheet to your page.
See the Sigil User Guide and its tutorials for details about Sigil.