The first card of any site is called Welcome Visitors. It is the card you see if you specify no other page in a site's URL. One click takes you to this card for any site.
The Card Flag is the large colorful square by the title of every card. All cards from a given site carry the same flag color.
Select the flag of any card to launch that site's Welcome Visitors card in a lineup with the card of interest.
When you Follow Links to cards leading to other sites (e.g. on other web domains or subdomains), you may see flags of different colors.
When you select a flag, you will regenerate cards starting from its Origin Server for your continued browsing.
When sending links to a card by email, consider following links from the beginning through the page of interest. This lineup provides the reader with context.