As Laryn said, email clients don't like iframes. This is because of the potential for malicious content, so if you use iframes in your email not only will they not function but they could flag your entire message as junk and lower your delivery rate.
Laryn's answer is the best answer for basic inclusion of a video in your email (link an screenshot to the video).
However, you might want to use the public archive of the Civi Mailing on your website, i.e. the link for joomla will be something like https://[yourdomain]/index.php?option=com_civicrm&task=civicrm/mailing/view&reset=1&id=[CIVI Mailing ID]. (That's also the "read in your browser" link that might show at the top of your emails.) And for that, you may want to embed the video rather than having a link. To do that --after the mailing has been completely sent! --, you would need to edit the code in the database to replace the linked image with the appropriate iframe. A careful layperson who knows something of html can do this:
- In your webhost's control panel (not Joomla, the host) use the utility phpadmin to access your database.
- Find the table civicrm_mailing and Browse
- Find the line for your mailing and Edit.
- In the body_html field, find the ...image here... tag of the linked image. Wrap those tags in the appropriate iframe like so:
<iframe [details of the embedded content]>
- Click "Go" to save your changes.
The point of wrapping the iframe around your original content is so that any browsers that don't support iframes will then show the original linked image.
If you do this, be aware that if you then reuse that mailing in Civi, you may have to deal with an inappropriate iframe. Not sure how Civi handles this.
Here's an example from our website. I've not only changed the body_html of the mailing to include the iframe, but I've embedded the actual mailing in a joomla article so it is better integrated into our website.