Impressions are the number of times a post was displayed on screen, counting every repeat view by the same account. A post with fewer unique viewers than another can still post more impressions, if people kept seeing it again.
Reach and impressions are usually read together: a large gap between the two says a post is being shown repeatedly to the same, smaller group rather than reaching new people.
A gap between reach and impressions that grows over time can mean a post is being resurfaced repeatedly to the same audience rather than finding new viewers.
The two numbers together are also how a post's true frequency shows up: a steady stream of impressions with reach barely moving usually means an existing audience is seeing a post again, rather than a new one discovering it for the first time. Neither number on its own says which of those is happening, which is why they are worth reading as a pair rather than picking one to watch.
A common mistake is treating a rising impressions count as growth on its own — it can just as easily mean a platform is resurfacing a post to the same people repeatedly, which reach next to it would reveal and impressions alone would not.
Impressions matter for planning mainly as the denominator hook rate is often measured against: without it, a strong view count says nothing about whether the opening seconds actually earned the attention they were shown to, or simply that a lot of people were shown the video at all. A single post can also post very different impressions counts on two platforms that define a view differently, which is why the number is read within one platform's own history rather than compared across platforms directly.
Where it comes from
The same Meta Instagram Insights documentation defines reach and impressions side by side, drawing the line between a unique account and a repeat view.
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.