Google Webmaster Tools (GWT) is a set of tools which make detailed reports on website visibility in Google. You should register on https://www.google.com/webmasters/tools, add your website, then select verification method ‘Add metatag to home page of your website’.
After selection of this method metatag will occur, for instance "<meta name="google-site-v verification" content="p80gS_iPNueHS2CVWVGCqIC9v6MedK2RsgFsbrC19Xs" />"
You should copy 'content', in this case it will be "p80gS_iPNueHS2CVWVGCqIC9v6MedK2RsgFsbrC19Xs" (without inverted commas) and paste in Administration settings.