The about:config page in Firefox and other Mozilla software lists a litany of default and stored preferences, some better managed through options dialogs, the rest hidden for advanced users. Tinkering with the settings in here generally requires you to look up what they are online. Mozilla does, however, have its own documentation for many of its preferences in source comments in the default preferences files. This addon lets you see them in about:config.
This extension provides a quick way to view the information about preferences that is already shipped with your browser. For more detail and information about other preferences without source comments, you'll still need to look them up online: