Capacity For Experience is an achievement quest. Once you reach a certain level for your character (in this case, Five Dragons 6 or level 66) you will receive this quest
Accepting the quest
You are not required to kill any mobs or do any tasks. All that is needed is to go meet with a Non-Player Character who is offering a reward for your character's level achievement.
For White Clans, the NPC's name is General Store merchant Gao Yilang (in Jinan). For Black Clans, go find Yan Taiyi (Datong). Look for the white exclamation marker on the ingame map.
Description as follows: Exp. boosts by 20% and Kung Fu Exp. boosts by 50% for a day
You will receive an item that will boost your Experience like an XP card. Right click on it to use and it will disappear from your character's bag.