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 traffic.
First Time Traffic Graph : The background of this graph shows the total daily traffic on your site. Superimposed in pink is the daily first time traffic.
Returning Traffic Graph : The background of this graph shows the total daily traffic on your site. Superimposed in blue is the daily returning traffic.
Important Note : by default, your account is set to 'test mode' in which a visit is counted every time your page is loaded. If the same visitor reloads your page 5 times in 30 seconds, then 5 visits will be counted (this is done to allow you to test the counter after installation). In order to get more meaningful stats, you should set your account to count 'visitor sessions' by selecting 'Type of Tracking -> Visitor Sessions' in the Control Panel. This ensures that a visitor loading your page several times in a row during the same browser session is only counted once (if that person comes back to your site 30 minutes or more after leaving it, a new visitor session will be counted).