An iframe loads HTML content from a different domain onto your website. Think of it as a page embedded in a page. The page served in an iframe has its own URL and operates separately from the rest of the page, so what happens in the iframe stays in the iframe.
iframe and email
How to know if your site uses an iframe
The most common scenario we've come across is with email capture popups. Another scenario might be within your checkout system. In general, if you use any external tools to serve an element of your page there's a chance it's serving through an iframe.
How to check if that page element is an iframe
What to do if it's an iframe
In order to read and collect visitor emails, your AdRoll Pixel needs to be firing on the page where the email is being submitted. Whether you have direct access to the contents of you use an external tool, you will want to add your AdRoll Pixel so it fires within the iframe.
There are a lot of tools that work with AdRolll and are free to use. For example: