Insights is a page of data showing your reach. It will graph your posts, people that are talking about the page and weekly total reach.
Like for me when I look at my insight page I see that I have a 5.66% increase in likes, 5.67% increase of friends of fans reached, 200% increase in people talking about my page, and a 23.42% increase in weekly total reach. It has been a good week for me. It will also show decrease in activity.
Below the graph you will see a list of your posts, reach, engaged users, talking about this, and virality.
I think the reason for this page is to help you see what things you post get the most recognition and such. For me it tends to be baby items that get great views and such.