From my understanding, this applies to the position in line of the MAS when forced induction is used.
For a "draw-thru" meter, the meter is IN FRONT of the blower/turbo. Having it here, the air is 'drawn-thru' as it is being sucked in by the impeller and then pushed into the motor.
A "blow-thru" meter has it in line AFTER the blower/turbo. So, the air is sucked in through the air filter, compressed by the impeller and pushed THRU the MAS on it's way to the intake.
I'm sure there are advantages and disadvantages for both but I am not sure what they would be.