Verifying your site with Google Webmaster Tools allows you to get additional insight into how Google views your site.
You only need to use the part of the tag underlined in the image above (i.e. just the part between the "quotes" after 'content'). Don't paste the whole thing in because it won't work!
You can also use Google Analytics to verity your site. The first step is to setup Google Analytics with your account, please click here for the instructions.
Once Google Analytics is connected, the next step is setting up Google Site Verification: Sign into Google Webmaster Tools, enter your Thinkific URL, and then choose Add a Property.
Now you will be presented with various Verification Methods, please choose Google Analytics, and finally choose Verify.