An impression is a single display of a link to your website in Google Search, Discover or News. According to Google’s explanation, an impression means a user saw the link or may have seen it. You find the number of impressions in the Performance report in Google Search Console.
When does an impression count?
In regular search results, an impression counts as soon as your result is on the results page that was opened, even if the user does not scroll that far. In a carousel or an expandable block, the result usually has to come into view or be expanded. With continuously scrolling results and in Discover, only what comes into view counts.
Example
Say someone searches for “gutter cleaning Utrecht” and your page is in position 7 on the first results page. The user clicks the first result and does not scroll any further. Your page still gets an impression, because it was on the page that was opened.
Common mistake
Reading impressions as the number of people who actually looked at your result. An impression is a display; it says nothing about attention. Look at impressions together with clicks and click-through rate. A rise in impressions without extra clicks can mean you appear for more queries, but lower down or for less relevant ones.
Why a page gets no impressions at all is covered in Why is my page not showing in Google?