Impressions are the total of a Member's friends and followers on their social networks. The Impressions generated by a Share will depend on which networks they shared the content, and how many followers they have on that network.
How do Impressions work?
If you have 500 Twitter followers, 200 Facebook Friends, and 100 people following you on LinkedIn, content posted to all of those networks will net 800 Impressions for that Share. If you choose not to share a post on LinkedIn, you may only get 700 Impressions for that post.
Impressions do not count everyone who has "seen" a post, it is simply the total number of followers on that network at the time of the Share. This means that some of the followers of a Member may not have actually seen that content. Impressions also doesn't count anyone who sees the content as the result of a retweet or reshare, which may be higher.