Experience is a way to gauge one of the many ways adventurers grow in capability. As experience is gained, one's overall ranking will increase which results in the gaining of levels. Experience can be gained through hunting denizens or creatures, completing quests, or slaying other adventurers.
Before the year 482 AF, experience was "capped" at the 100th circle of experience. Now, adventurers may gain a "potentially limitless amount."