By default, each content file name that you create in Drupal is named as ?node=xx. If you are creating content that is easy for your users to understand when someone looks at the URL or for search engine optimization boost, you will want to consider having a descriptive file name like http://www.domainname.com/my-keyword. To enable this capability you want to enable Clean File URL from administer » site configuration » clean URLs. The path module should be active for this feature.
Once enabled, Drupal allows you to substitute or alias an automatically created file name with a descriptive filename. This is great for giving descriptive names for taxonomy terms and creating dynamic content that we will go through in the advanced modules. It is recommended to set this feature at the start of your installation, as your file link name will change if you set this after the bulk of your content has been created. For each content you create you will have the option to enter a friendly URL name. You can also change the friendly name by reviewing the list of aliases from administer » site building » URL aliases
If for some reason the test for "Run Clean URLs" fails, follow the link in the resources section to resolve this issue. For the most part as long as your hosting provider gives you the ability to control your .htaccess the "Run Clean URL" test should succeed. Get the help of your hosting provider if you need assistance.
In this video you will see how to enable clean file urls.