Affinity is a mechanic of the mod.
Changing Your Affinity
To change your affinity, you will need to find an Affinity Tome and right click it. This will consume the tome. They spawn naturally in Mage Towers and Mage Archives. They can also be crafted in a Spell Research Table. Researching an affinity with no spells left unlearned for that affinity will create an affinity tome for the same affinity. A "None" Affinity tome can be obtained the same way using Purified Vinteum in place of essence. Affinity Tomes still remain a finite resource, as they require a Lost Journal Fragment to create.
Having an affinity grants a reduction in mana cost to spells that share the same affinity. There is also the advantage of being able to cast augmented spells in your chosen affinity without needing a reagent.
All spells from your opposing affinity, however, have their mana cost increased and reagent costs are doubled.
There is a tome that can be found which will clear your affinity rather than replace it with a new one.
You lose your affinity if you die, though this can be turned off in config.
Your affinity is represented by an essence icon to the left of the player action bar in-game.
Spells by Affinity
Spells are organised here by affinity, to help you decide which affinity to use. Unless otherwise specified, the reagent for augmented casting is the same as the affinity.