The Term "Safe/Regular iFrame"
Generally an iFrame (Inline Frame) is an HTML document that is embedded inside another HTML document on a publisher website. iFrames often insert content from another source, and in our case are commonly used to serve advertisements.
Now, the difference between a Safe iFrame and a Regular iFrame all depends on the source domain of the publisher page and of the iFrame.
If the domain of the iFrame and the Publisher page are identical or allow cross referencing this is known as a Safe iFrame as content from the iFrame can be injected directly into the publisher page.
If the domain of the iFrame and the Publisher page are not identical or do not allow cross referencing, this means that content from the iFrame cannot be injected into the publisher page.