How You Reached People is one facet of Facebook's Insights (analytics) program. Facebook breaks down how people were reached by a Page’s post by categorizing the interaction as resulting from Organic (in the newsfeed because the user Likes the Page), Paid (the result of an advertisement) or Viral (in the newsfeed because someone else in their network interacted with the post).
Why do you need to know How You Reached People?
This analytics tab lets you know how, and how often, fans are actually seeing your Page posts. This insight helps marketers understand which types of content perform best with their target audience.