When someone comes to your site there are two possibilities : either that person has never been to your page before or he/she has come at least once in the past (whether a few hours ago or several months ago). In the first case, we speak of first time traffic (it's the first time the visitor ever sees your site), in the second case, we speak of returning or repeat traffic.
Each bar represents one day, and the bar's height represents the number of [ visits ], [ first time visits ] or [ repeat visits ] on your site during that day (the exact number can be read at the bottom of each bar or by hovering your mouse over the bar).
Saturdays and Sundays are represented by slightly lighter colored bars
If the period you are looking at is greater than 60 days, a red line will appear superimposed to the graph. It represents the 28-day rolling average of your repeat or first time traffic. Since it smoothes out the daily variations it gives you a better idea of your long-term traffic trends.
Note that to recognize repeat visitors we depend on cookies on the visitor's browser. If these cookies are not enabled or if they get reset, a repeat visitor will not be recognized by 123Stat and will be inacurately identified as a 'first time visitor'. This is a limitations of the system ; unfortunately there is no better way of measuring this metric and all the stats provider are subject to the same issue.