Spiritual Cultivation is a measure of how well you understand the world. With an increased cultivation level, you are able to learn new and more powerful skills. Cultivation level is increased by completing specific quests, commonly known as Culti Quests, which culminate every ten levels starting at Level 9. When you are ready to advance to the next cultivation level, you will receive the Spiritual Cultivation Guide quest, which will direct you to the NPC who will provide the final challenge.
See Quests by Level category for lists of quests, including Cultivation Quests