How do “impressions” differ from “reach”?
Impressions measure the number of times your post or ad appears to users. It makes an “impression” on a viewer but this metric doesn’t take into account unique users. That means that if the same person views it 20 times it counts as 20 impressions.
The metric of “reach” measures the number of unique users who view your post or ad. In that same situation the 20 views by one person would count as a reach of only one.
Here’s a post on blog.hubspot.com that focuses on how both metrics work on Instagram.