To protect your Help Center from potentially unsafe content, Kayako, by default, doesn't allow certain files or code snippets in your articles. However, if you're adding a video or other kinds of multimedia, you can choose to ease those restrictions so your embedded media will be displayed.
To do this, you'll need to go to the admin area, click the Security Policies link in the sidebar. Navigate to the Help Center tab and then check the Allow potentially unsafe HTML in Help Center articles box.
With this option checked, you'll need to take extra care to make sure none of the content or code you're embedding could be exploited as a security risk.