The groundhogg-lead-source cookie is automatically added for all new visitors to the site.

What this cookie does is simply keep track of where the visitor came from.

For example, if you click a link from Facebook.com to your website, the groundhogg-lead-source cookie will be set to the value of Facebook.com

It does this simply by accessing information that is readily available from the browser. This information does not come from the server, but rather directly from the browser via javascript. Thus the visitor’s data never touches your server.