Here are page post insights terms as defined by Facebook:

• Reach is simply the number of people who have seen a post on your page.
• Engaged Users are the number of people who clicked anywhere on your post.
• [People] Talking About This is defined as the number of people who have created a story from your Page post (Sharing, liking or commenting, answering a question, responding to an event, etc).
• Virality is the number of people who have created a story from your post as a percentage of the number of people who have seen it.

Businesses can use Facebook insights to identify content that effectively engages their customers, expending more effort on topics that their fans have interest in and reallocating resources away from content that isn’t as effective.  They will also benefit from analyzing how well their page posts are reaching their target market and how they can improve their content to reach the demographics they’re targeting.