With an iFrame you can embed external documents or applications. You do this using a URL or link, which is then displayed in the iFrame element.
Examples of use include audio and video players (Soundcloud, Twitch, Dailymotion, etc.), a blog, appointment calendar, booking form or currency converter. A PDF can also be embedded in a roundabout way using an iFrame (see here). Of course, there are countless other possibilities.
Here you can read how to insert an iFrame.
Here is a website in Website Creator, with examples of how to use iFrames.