An impression is counted whenever a page shows up on a search results page that a user actually views, whether or not they scroll far enough to notice it or click anything. Reaching page two of results still counts as an impression the moment someone opens page two; a page ranking well on page five that nobody ever loads gets none.
Impressions are a visibility metric, not a performance metric on their own: a page can rack up thousands of impressions for a query it ranks poorly on and still generate almost no clicks, since most searchers never scroll past the first handful of results. That's exactly why impressions are most useful paired with click-through rate and average position rather than read alone. A sudden jump in impressions for a query usually means a ranking improved or search volume for that term rose, and checking which of the two happened, along with whether clicks moved in step, is what turns the raw number into something actionable.
