By default, Backdrop uses and generates URLs for your site's pages that look like http://my.site/about-us. If for some reason the so-called "clean URLs" cannot be enabled, this would be displayed with ?q= as http://my.site/?q=about-us.
The style of URLs using ?q= can be hard to read, and may even prevent some search engines from indexing all the pages of your site. Research suggests this may not be as big of a problem for major search engines as it once was; however, it is worth noting the recommendation from Google's webmaster guidelines: Keep a simple URL structure stating:
Overly complex URLs, especially those containing multiple parameters, can cause a problems for crawlers by creating unnecessarily high numbers of URLs that point to identical or similar content on your site. As a result, Googlebot may consume much more bandwidth than necessary, or may be unable to completely index all the content on your site.
If you are not seeing clean URLS on your site, read about how to enable clean URLs.