Reach is the number of unique people who may see these posts. Impressions are the total number of potential views (including same users seeing multiple posts).
For example, if you have 500 followers and share 2 posts, your reach would be 500 but your impressions would be 1,000.
If you think the number is wrong, the first thing to look at is posts and users in your dashboard. If these numbers are equal, your reach and impressions will be equal.
If you have 500 followers and share 1 post, your reach would be 500 and your impressions would be 500.
When people start sharing multiple posts, your impression will change. As a simple example, we will say each of the 500 people shared the post twice.
So, if 500 people each with 500 followers share a post once the reach would be calculated as 500 people x 500 followers = reach of 250,000.
Impressions, however would now be 500 people * 500 followers * 2 posts = 500,000 impressions.
To learn more about our Exposure metric, please visit this answer.