As you level up and do combat with your fairy, the Communion bar will increase. Once it's filled up you can make use of the Extract function to create a Skill Stone for her.
Once a day you'll receive a one-off free entry to the Abyss for the chance to win great loot in a timed event.
Keep in mind the game uses an energy system which limits how often you can use your powerful abilities.
Passive skills are powerful upgrades that are permanently enabled, while Active skills (as the name suggests) need to be activated from the combat "wheel" that surrounds your basic attack button. These use up precious energy, so use them cautiously and always have a move or two available for a surprise encounter.