The total number of times a piece of content was displayed, including multiple views by the same person.
Impressions count display events, not unique viewers. If one person scrolls past a post three times, that's three impressions and one reach. Impressions is the inflated, marketing-friendly version of reach — useful when you want a bigger number, less useful when you want to know how many distinct people saw the content.
For ad spend math, impressions are the unit advertisers buy (CPM = cost per thousand impressions). For organic content analysis, reach is more honest.
A high impression-to-reach ratio (e.g. 5:1) signals viewers are coming back to re-watch — usually a sign of high-quality content or strong rewatch behavior on short-form. A 1:1 ratio means nobody is re-engaging.