Reach is the total number of people who see your content. Impressions are the number of times your content is displayed, no matter if it was clicked or not.

Think of reach as the number of unique people who see your content. In a perfect world, every one of your followers would see every piece of content you posted.

So an instagram post

might have the following insights

So for this example - the post has been seen by 696 unique accounts (reach) and a few of them have seen it more than once, hence the impressions are a bit higher at 701.

Typically impressions would be about 8% higher than reach.