Iframes allow you to visualize in the page a content coming from another web platform. It is thus common to add a Google map via this protocol, the map being hosted remotely on Google's servers but "encapsulated" on your site. Another very common example is the video included in a press article.
For this, you need to copy the "embed code", which is a code framed by iframe tags.
In your block editor, go to the "test" tab, which is the coded version of what you see in "Visual".
Paste the iframe in the desired location and save.