The difference in one example
Reach counts people; impressions count views. If an ad is shown to 10,000 different people and each of them sees it twice on average, reach is 10,000 and impressions are 20,000. Impressions are always equal to or greater than reach. Platforms report both because they answer different questions: reach says how many people you got in front of, impressions say how much exposure you paid for.
Divide the second by the first and you get average frequency, the figure that warns you when the same people are seeing the same video too often. The ad frequency entry covers when that starts to hurt.
Which number to report
For awareness goals, reach is the honest headline, because a million impressions served to the same small group is not a million people aware of you. For cost, impressions matter, since most ad buying is priced per thousand of them; see CPM.
Be careful when adding up reach. Reach on Instagram and reach on TikTok cannot be summed into one total audience, since some people saw both, and platforms define a counted view differently. Report each platform separately or label the sum as an upper bound.
Reading creator post reports
When a creator shares post insights, ask for reach, impressions and the share of viewers who were not followers. A post with high impressions but low reach was seen repeatedly by a loyal core; a post with high non-follower reach travelled through recommendations.
Ask for screenshots from the platform's own insights rather than typed numbers, and agree in the contract which metrics the creator reports and when, so the comparison across creators is fair.
How it's used
The post reached 42,000 accounts but logged 61,000 impressions, so a fair number of people watched it twice.
Please send reach and impressions from your insights seven days after posting, as screenshots.