A portal direct link is the URL from within the embed code that links directly to an image, video, document, etc. There are two types of direct links: one for the portal itself and the other for an asset within the portal. The portal direct link will take anyone you share it with directly to the portal, while the asset direct link will give anyone it's shared with an image of the asset.
For example, for the asset below that's included in a portal, the embed code is a snippet of HTML that can be pasted into an email, blog post, etc. It looks like this:
The direct link from within the embed code is: