Landing page is the page of a workspace where all users are directed with email links (if not changed before sending the link).
Every workspace has to have one, since some pages can have limited visibility.
Admins can change the landing page in two different ways:
Go to the page that will become the new landing page and click Action menu > Page settings > Set as landing page. Now the page is the landing page.
Or go to the Pages tab Action menu > List all pages, find the page that will become the landing page, click the ...-menu at the right side, choose Edit and Set as landing page.
From the same Pages tab it is possible to see which page is the current landing page by locating the anchor icon.