Both metrics are similar, but there is a difference when it comes to engagement. In general, your impressions will always be higher than your reach. If a large number of followers see it again, the number of impressions you get might be close to or may even be higher than your follower count.
What happens after 10k followers on Instagram?
Once you have 10k followers, Instagram will make it easier for you to get people to your website via Stories with the swipe up to link feature. Swipe up is the only way to get a direct link from your Instagram to your other web properties. And, it’s available in STORIES, if you have 10k followers.