Organic reach measures how many users have seen the post on their screen. Previously the number of users who got the post in their news feed was measured – whether it appeared on their screen or not.
The average (mean value) is a good figure for the newsroom/department to talk about at weekly meetings, for example. It provides a concrete, easy to understand picture of the trend over time for an account on social media. The average (for reach, comments or shares) is also an interesting starting point when you compare your numbers with those of other similar pages.
Read more about Averages and how Facebook Insights works here