Impressions count display events, not unique people. If a single user scrolls past your post three times, that is three impressions. The unique-people equivalent is reach.
Impressions are the headline volume metric for paid advertising and the primary metric for top-of-funnel awareness campaigns. They tell you how many opportunities your content had to be seen — not how many people actually saw it, and not how many engaged.
Compare impressions in context, not in isolation. A 10× increase in impressions with flat clicks usually means the algorithm pushed your content to a poorly-targeted audience. Cost per impression (CPM) is the standard normalisation when comparing paid placements.