Player EXP are points you get from completing Quests, Monster Hunting, diplomacy, attacking an army in a rally or being attacked by other players, etc. It is used to level up the player level, which is something that allows you to equip higher level gear and increase heroes level (the max hero level is the player level and max player level 60 for now). It also gives you talent points.
After level 56, the amount of points you need to increase player level becomes much higher than it has been until then.