If you're using Instagram for you business, it helps to know what the word "impressions" means.
Hubspot said, "Impressions measure the number of times your post or ad appears to users."
Seems simple enough!
However, you also have to consider if people are seeing the post multiple times. If the same person saw your post five times the app would still count that as five impressions.
If your focus is trying to reach new potential customers, look at the reach metric instead.