An iframe (short for inline frame) is an HTML element that allows an external webpage to be embedded in an HTML document. Basically it's a website within a website. It allows you to insert a piece of content from another source into a webpage.
The source code for an iFrame will look similar to this:
<iframe src="http://example.com/blog.html" width="500" height="90"></iframe>
So for example, the popup (outline in orange below) is content being shown from another webpage using an iFrame. Hopefully this makes more sense now :)
How to Capture from an iFrame:
If the email input field on your webpage is embedded using an iframe, then you'll need to follow these steps to capture conversions.
How to fix
Right click on the area of the page being generate by the iframe > select 'View Frame Source'. You may need to click around a few times until you find the area of the page that contains the iFrame.
Copy the URL from your browser's address bar. Make sure to exclude
view-source: when copying the URL.
Paste the url into the Capture section of your Proof campaign (third step during your publish campaign process). Please make sure that the pixel is also installed on this new capture url so that your notification will be able to display.