One of the metrics it lists at the top of the summary section is 'Pages with first impressions (estimated)'. But what does this mean?
Thankfully there's an accompanying explanation from Google themselves, if you dig hard enough:
Pages with first impressions: Pages that were seen by a user in Search Results for the first time last month. (This may be omitted for very large sites.)So, these are pages that have only started appearing in the search results this month - they may indeed be old pages that simply haven't appeared in search results as they are not sufficiently well ranked to appear to end users themselves.
And to be clear 'impressions' means that the results have been shown to a user in their Google search results, not that they necessarily have clicked on them.
The reason why could be a combination of factors: very specific searches meaning that page is a relevant search result; tweaks to the pages themselves; updates to the Google algorithm; changes to the overall site ranking that affect the likelihood of one of the site's pages appearing above a competitor.
It does lead to the question of which pages these are precisely. Unfortunately, there's no link to a corresponding page in the Google Search Console even though the rest of the report features a number of links offering more detail on the monthly email overview of performance.
In terms of the overall report, if you're puzzled as to why you are seeing this for the first time, then not only is it relatively new in 2019, but also you'll only receive it if the site has been registered with Google Search Console for three months or more.
On the above image, the other metrics are simple enough. 'Impressions' is the number of times someone 'saw' your page listed in the search results and 'web clicks' is the number of times someone clicked on that listing and visited your site.