For a full list of embeddable content, check out Iframely. NodeBB comes with several other plugin options for embedding content as well -- if you have any questions please visit our community support forum.
This is what a topic and post looks like. As an administrator, you can edit the post's title and content.
To customise your forum, go to the Administrator Control Panel. You can modify all aspects of your forum there, including installation of third-party plugins.