What does the web source rule do?
The web source rule shows a lightbox based on how the visitor came to your site. You can pick direct traffic, referring website, Google Adwords, or organic search.
Use this rule to show a special message or offer to visitors who came to your site through your adwords campaign. Treat your visitors differently based on where they came from. AdWords traffic is expensive so offer an irresistible deal. Organic traffic might just need a nudge.