Question about Impressions metric

Basic question - when the Google Search Console gives data on Impressions - what is it actually telling us?
Is an 'impression' where someone has "seen" your listing ? Or is it that you FEATURE in a search but they might not have scrolled to the page you are on?

For example - if I search 'plumber London' and I pick a guy on page 1. Do they guys (and gals) on pages 2,3,4,5 get an 'impression'?

For your plumber in London the question is no. Only first page results will get an impression counted in GSC and the guy you click on gets also the click.

An impression is any page displayed/rendered on the users-side.

For example, if for your "plumber in London" search you scroll down on the search page and then click "next page" until the 10th page, all results present on the pages displayed to you will get an impression. Regardless if they come into your view or not.

@Chris.Roark much appreciated for clearing that up. That was what I was thinking though that does leave me with a number of pages that have poor Click-rates.