iFrames are useful, however they can be a bit hard to set up sometimes. By default you cannot embed any page since this can pose a security threat. This is why most pages prevent embedding and for some pages you need to use special embedding URL to get the embedding working.
Here are couple of examples how to use embedding and when to know when it is prevented.
- When you add a url to the iFrame widget, if you cannot see anything its not working :) Example would be www.gmail.com.
- Check if the page you try to embed has a Share or Embed buttons, from here copy the embed URL to the widget. See YouTube embed URL as an example below.