Horizon's Content page will display icons on files so users can quickly understand certain properties at a glance.
The hourglass means that the file is scheduled to self destruct. On desktop, hover over the icon to know its exact self destruction date in your local timezone.
The lock means that the file is locked. It cannot be trashed until it is manually unlocked from the Manage menu.
The key means that the file is server side encrypted. It requires a decryption key (?k=<key>
in vanity or CDN URL) to view.