This tab gives you an overview of how your store is doing. One of the first things on this tab is your Sales Statistics which will break down certain sales numbers for the day, week and month.
Underneath the Sales Statistics, you'll see the "Buys and Sells by Day" chart which graphs out your sells and buys (purchases). You also have the ability to drag and re-size the slider below the graph to see different dates and date ranges.
On the right side of the Overview Tab, you will see Store Totals for all time. You can also click on the totals to get more information about them.