What it means
One impression is one showing. If the same person scrolls past your post three times, that is three impressions and one person. Impressions are the raw count of times your content was rendered, which is why the number is always the largest one on the dashboard.
Why it matters to a small business
It is the denominator under most of your other percentages, so misreading it quietly bends everything downstream. It is also the number most often quoted at small businesses by people selling advertising, precisely because it sounds enormous and asks nothing of the buyer. An impression is not a view either: platforms count the moment your post was rendered on the screen, which on a fast scroll can mean nobody registered it at all.
A post picked up 5,000 impressions from 1,800 different people.
5,000 impressions divided by 1,800 people
A frequency of about 2.8. The average person who saw it, saw it nearly three times.
The bit people get wrong
High impressions with flat reach means you are showing the same small group the same thing over and over. That is not exposure, it is wear, and past a certain point it costs you goodwill with the exact people who already like you. Always ask for reach alongside it: a proposal that quotes impressions and leaves reach out has usually left it out for a reason.