The HTML widget code is used to add a campaign sign-up box on your website so that users can sign up to receive additional email campaigns from you.  The widget page gives you options for the font, colors, and size for the widget.  You simply copy the generated HTML code into a web page on your website to add the campaign signup.

Where do I find it?

You can get the HTML widget code for your Email Marketing Campaign by going to Services > Email Marketing: Campaigns, clicking the Campaign's Name and clicking Get Widget HTML.