This is a default page provided when you install QGIS Web Client. The default installation assumes that you checkout the QGIS Web Client from github into the directory "/home/web/qgis-web-client". To start with the client configuration, please copy one of the "site/js/GlobalOptions.js.templ" template files into "site/js/GlobalOptions.js". If you use different paths, please adjust them accordingly on this landing page (site/index.html) and in the file "site/js/GlobalOptions.js". You will also have to install QGIS Server (FCGI) and Apache with the FCGID module. You will have to configure Apache for FCGI use any your document locations.
This client is based on OpenLayers, ExtJS and GeoExt. It should be installed along with QGIS Server.