I could be wrong about this, but I believe the little number (with the arrow next to it) represents the one or two hour period with the most pageviews in the last two days. You'll notice as time goes by that the bars shift left as new stats are accumulated on the right. If a new hour has a higher pageview count than any already on the chart, the number will change. If no new hour has more pageviews than one already on the chart then the number will stay the same as that bar (the tallest on the chart) slowly moves left. Once it gets all the way off the chart, the number will change to reflect whatever the next highest hourly pageview count is.