What does it mean to maintain a positive g loading?
Short Answer

In straight and level flight, it means don't push over too fast.  Doing so gets you "light in the seat" and things may fly about the cockpit/cabin.  In some aircraft, negative "g" can make the oil "flow up" away from the sump and begin to starve your engine of much needed lubrication. (Example: T-34C, trainer, limited to 10 seconds inverted flight.  Straight and Level inverted flight is -1 "g.")

More Answer

You want to avoid flying with negative "g" in a variety of circumstances,  depending upon how the aircraft is designed.  


Example: In a helicopter, negative "g" can have dire consequences. 
On the teetering, semi rigid rotor heads in the original Huey, for
example, unloading the head can lead to mast bumping, which can
lead to catastrophic failure of the mast, and then some death. 
Counter Example: Flying a Pitts Special, or an F-18, the aircraft is
built to withstand some negative "g" due to their needing to fly
particular maneuvers.


"G" is a natural part of flying, but "how much" varies with design.

Whenever you turn the aircraft, you will add a little bit of "g" load since "g" is a measure of acceleration.  Vector sums based on angle of bank will tell you how much "g" you can expect in a level turn at constant speed.  


Example: a 60 degree angle of bank adds an additional "g" compared to
level flight, and you feel a bit of a push into your seat.