To embed videos e.g. from YouTube or other sites, it is recommended to use the Media element instead of the Flexible content, because it generates optimized markup and also cares about the user privacy.
The Flexible content element
The Flexible content allows you to place HTML source snippets in your page. These can be useful e.g. to embed external widgets. Of course, you can also use the Flexible content to add special markup for your page that is not available from the other elements.