He's referring to a page on a site for a product or service that is not yet available to buy/use/sign up for but that might do one or many of these things: collect e-mail addresses for a mailing list, collect e-mail addresses for future invites to an app/site, show "sneak preview" images of the future app/site, merely say "coming soon."

Going back a few years, though, "landing page" almost always meant a page that people are directed to (usually through advertising) that then attempts to "convert" them into a customer or subscriber of some sort, though this is not necessarily (but can be) a goal of the modern startup-type landing page.