A "navigational query" is a query with a certain website in mind. So like if you search for [facebook], 99.99% of the time you are looking to go to Facebook, if you search for [hacker news] 99.99% of the time you are looking to go to news.ycombinator.com. Basically it replaces the address bar for people who don't know what URLs are. You aren't looking for information about the business, you want to go to the site/business itself.