The number of times your material was seen by people, whether it was a post or a story.
Impressions, which are sometimes confused with reach, are the total number of times your material might have been seen. If your Instagram impressions outnumber your reach, it means your followers are seeing your material numerous times. For a good indication of what's working, look for posts with a high impressions-to-reach ratio.
When you want to focus on growing awareness among your target audience, Instagram Impressions might be a useful KPI.