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 :)
Now let's learn how to get the Capture url from an iframe.