Reach is the number of unique accounts that saw a post at least once. Someone who saw the same post three times still counts once.
Reach answers "how many people"; a rate built on top of it, like engagement rate or save rate, answers what those people did once they saw it.
A post can reach a lot of people and still engage very few of them, which is why reach on its own answers only the first half of "did this work."
Reach by itself does not say whether a post was any good, only how many people had the chance to find out. A small account with a low reach can still have a strong engagement rate on the people it did reach, which is why the two numbers are read together rather than either one alone — reach for who saw it, engagement rate for what they did once they had.
A common mistake is treating a low reach as proof a post failed — a small account can post a low reach on every video by definition, and the more useful question is whether reach is growing relative to that account's own recent history, not against an absolute number.
Reach also says nothing about which accounts were reached, only how many — a post that reached far outside an account's usual audience can post a strong reach and a weak engagement rate at the same time, for reasons that have nothing to do with the video itself. Reach is worth tracking as a trend across several posts rather than reading any single one in isolation, since one post's reach can swing on a platform's own distribution as much as on the video itself.
Where it comes from
Meta's own Instagram Insights documentation defines it this way.
Reach is the number of unique accounts that have seen a post or story at least once.
Read the source: Meta Business Help Center — About Instagram Insights.
What Rankfield reads
The landing page answers this directly, in its own words:
Your account end to end, your competitors, and the wider market around you: posts, videos, captions, comments, rhythm, what you offer. It watches the videos, not just the numbers under them.