In one line
How many different accounts saw a post, as the platform counts them.
A Mumbai brand reads the reach of a creator's Reel from a screenshot of its insights: 18,000 accounts saw it.
Reach is not sales, and it is not impressions: one account that sees a post three times counts once in reach and three times in impressions.
Agree: which platform figure you will use, the date it is read and how it is shared with you.