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.