An iframe is an HTML element that allows one website to display content found on another website.
Example iframe code:
Similarly to rich media types, search engines can use iframed content to discover parts of the web they’ve never seen before, but they don’t treat it like normal text-based content.
It is theorized that Google will use iframes to find content (for expanding the index), but won’t attribute the content’s value to the site is framed into.
So, use iframes sparingly and if you want the content within the frame indexed and counted toward your domain, don’t use an iframe.
This is honestly one of those things that doesn’t come up too often anymore, but depending on the age of the site you’re working with, you may see it (very, very old websites may be built using only iframes).
To learn more about iFramed Content (how to properly and improperly implement, as well as how to find it on your site), signup for SEO Pro here