A landing page is simply a page that you want users to land on when they first hit your website. In essence, every page on your website is a potential landing page!
When it comes to SEO, a landing page is the page you want a customer to land on when they search for a particular keyword in Google.
For example, let's say I want the Hike website to appear when someone searches in Google for "small business SEO tips" I want Google to show the searcher the Hike blog page (not the homepage, or the pricing page, or the terms page, etc...).
Therefore, the Hike blog page is the landing page for the keyword "small business SEO tips".
So why is it important to add these into Hike? In order for your landing page to appear in Google for the terms you want it to appear for, you need to optimise that page specifically.
By assigning keywords to your different pages Hike will then tell you - within the actions section after on-boarding - exactly how to SEO those keywords on those pages.