Some websites allow you to embed their content on another page via iFrames. Our technical and security advice is that this is not a safe ands secure thing to enable on Glow Blogs.
Many of these types of content, YouTube, Google Maps, twitter also supply oEmbed codes and these do work in Glow Blogs. For example if you just paste the url to a youtube video into a empty line in a post the video will be embedded. The Jetpack plugin extends the content you can embed see: Shortcode | Jetpack for some more details and examples.
we have also developed embedding for other types of content that depend on iFrames, see the Embedding media page for details.