Set 1 are the readings for that trip (including any short breaks) and set 2 are the cumulative readings since the counters were last reset. So if you set off in the morning say, and then stop somewhere for 10 mins and then drive for another hour, set 1 will show the readings for all the driving you've done that day. If you stop for more than a certain amount of time, then the set 1 readings will start again from zero as the car thinks you are starting a new trip.
I guess a lot of people will reset the set 2 readings when they fill up with fuel.
Hope that helps!