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.
For example, if I have 500 Twitter followers, 200 Facebook Friends, and 100 people following me on LinkedIn, content posted to all of those networks will net 800 Impressions for that Share. If I choose not to share a post on LinkedIn, I may only get 700 impressions for that post.
Impressions does not count everyone who has "seen" a post, it is simply the number of followers. 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.