Has been covered before. I'd say "do a search" but it's down!
mpg 1 = average mpg for the journey (and includes any journies within a two hour period*)
mpg 2 = total average mpg
mpg1 gets reset if the car hasn't been used for more than two hours, so if you do lots of short trips with say five mins in between, it'll still show the average for the journies.
mpg 2 doesn't get reset until you reset it.
Press and hold the button on the bottom of the stalk to reset.
I'm sure this info can be found in your car manual (I guess they put it there for more than just show), and if you don't have one, I recommend you buy one!