Google Webmaster Tools is a service that is used to send a sitemap to Google, see information about the traffic to your website, which search frases are used and potential errors on your website.
Log in to Google Webmaster Tools
Log in to Google Webmaster tools with your Google account and click the "Add a site" button. Once you have filled out your URL, choose the tab "Alternate methods" and click "HTML tag".
Copy the meta tag and paste it in your shop admin on the Settings > General settings > Advanced settings page and "Save". The page cache needs to be updated after this, go to the Settings > General settings > Page cahce page and click "Update immediately".