PG is an abbreviation of Power Game, also called PGing and power gaming .
When you PG you focus on getting high exp/h, this can mean that you skip looting, loot less (such as only pick up high value items, called rares) or simply go to a spawn where you don't receive very much loot but get a lot of exp.
Some use the term PG for when they do long hunting sessions and in this case it might just refer to getting a lot of exp but not necessarily a high exp/h. This type of use of PG is however technically not the "correct" use of the term.