If a domain or page started ranking within the top 50 organic spots (but wasn’t last month) on a search term, that is considered a new keyword.
When you search a URL’s history, you might see keywords that aren’t new to this domain, but they are new to the specific page that you searched.
Further in the SEO Overview this is shown as “Newly Ranked” in a chart. This number tells you how many of the domain’s keywords fall into this category. If you click through, you can create a special report that details exactly which pages rank for these keywords.