What does "navigation link" mean?
The navigation link is the clickable text in the navigation bar at the top of your website that allows you to go directly to a page without scrolling to it. Selecting "Yes" or "No" determines if that page will have a direct link in your navigation bar. You can change the name of that link by typing a new one into the text box next to the drop down box and saving.
It is possible for a page to be in the layout of your site without having a link to it in the navigation bar. Some pages can be scrolled from top to bottom, so your guests will still see the page if "No" is selected, but the page is not completely removed from your layout.