The center panel shows your mail displayed. The headings From, To, Date and Subject display mail information as is described by each of the labels; for example, From shows the sender.
Sorting your mail
The mail list headings are actually buttons which allow you to instantly sort according to that heading. For example, clicking on From will show all your mail arranged in alphabetical order according to the sender's name. The same sorting feature applies to all other heading buttons, Date is, of course, in chronological order with most recent first and then clicking again reverses the order with most recent shown last.
Sort in reverse order
Clicking a second time will reverse the order.