The "landing page" is the web page on Leanpub where people can buy your book, bundle, course, or course set.
You can find the landing page for your content in a couple of ways.
First, you can go to your author dashboards:
https://leanpub.com/author_dashboard/books
https://leanpub.com/author_dashboard/bundles
https://leanpub.com/author_dashboard/courses
https://leanpub.com/author_dashboard/course_sets
If you click the cover image on anything you see in a dashboard, you will go to the landing page.
You can also find a landing page if you search for it, using the search feature at the top of most Leanpub pages, like our homepage at https://leanpub.com.