The subtitle appears below the title of the page. It defaults to the theme's highlight color, but you can change it for all Portfolio Items on the Portfolio Page, under Page Options > Page Subtitle Color.
If you choose to enter one, the intro will appear before any entered main content. It appears bold and at larger font size than the regular content.
Here you can enter a Youtube or Vimeo URL. Note this will overwrite the featured Image, but you may still wish to have a Featured Image as it will appear as the thumbnail on your main Portolio Page