Leveling

Although characters start with a minimum of 1 Kavana, over the course of an adventure they will most likely grow in power and divinity, being represented by an increase in their Kavana. 
Whenever characters gain Kavana, they increase their maximum Condition by 3 + PHY for each Kavana gained, and can then spend the Kavana gained in one of two ways:
#  Gain one feature from the various ability trees, as explained in the Ability Trees section, with each features costing 1 Kavana respectively;
# Gain attainment and learn Magical Effects in a certain Way of Magic, as explained in the Magic section, learning 6 magic effects at the cost of 1 Kavana.
Both feature mechanics are further explained in their respective section, but overall, all features and attainments always cost 1 Kavana, making not spending or hoarding Kavana unnecessary.
Once enough Kavana is obtained by the characters, they may go up in Stratum, as detailed in the Stratum section. In summary, Stratum is divided between four tiers. From lowest to highest, these are: Mortal, Exalted, Demi-God and Empyrean. Once a character advances to a higher Stratum, they gain access to new abilities and attainments not previously available, certain skill proficiencies, and gain access to their character’s Ace Ultima, as described in the Ace Ultima section.
Gaining features and magical attainment may also add cards to your Gambit as a consequence, with those rules being found in the Deck and Gambit section of the rules.
If you are still leaning the rules, advance to the Ability Trees or Magic section of the rules. Followed by the Stratum or Ace Ultima section of the rules.