Here are definitions of some of the terms used in the Analytics area:
Engagement: A person saw your post and did something with the post. They may have clicked on it, liked it, shared it, commented, etc.
Reach: The number of unique people who had any posts from your Page enter their screen.
Impressions: How many times this post was shown on a news feed – a post could be shown more than once for an individual. Impressions will always be greater than or equal to reach.