Pure Mage Levels, or PML for short, are a secondary system of spell scaling beyond mere caster level.
In addition to the base damage of a spell based on CL, arcane spells may also be granted further bonuses dependent on how many levels of raw spellcaster classes you may have, as a way to make up for a 30 wizard or 30 sorcerer not having as much outside of casting as a cleric, shaman, or druid might. For example..
Wizard, Sorceror, ASoC, Palemaster, Demonologist, Mystic Theurge, Golem Master, Cerebremancer, and Red Wizard are examples of caster-focused, non-gish classes. Lich, Brachina, Grey Slaad, Rakshasa Sorcerer, and Succubus are examples of applicable RPRCs.
For every three levels in an arcane caster-focused class, you get a bonus to the damages dealt by your spells. This bonus, most of the time, applies additional +1 CL worth of damage dice for that spell, but can provide other benefits as well. For example..
CL 30 Fireball would deal 30d3 damage base and before metamagic. For every 3 levels in one of those classes, you would gain 1 "CL" worth of extra damage dice from taking those pure mage levels, in fireball's case. So a Level 30 wizard would have 30d3 + 10d3 for a fireball.
What this does, is reward players who forego the usual gish options to play a more caster-heavy role, and allows for spells to be heavier-hitting for those builds who have little else to do. Vanilla NWN2's fireballs did 35 dmg on average, for reference, whereas a 30 Wizard would deal 80 before any metamagic.
An example of other benefits not related to damage would be Bigby's Forceful Hand lasting for longer durations based on PML, or Combust having extra +1's to DCs added to the "put out the fire" save. These benefits are typically rarer but will always be listed in the related spell's description.
NOTE: PML also applies to ANY use of a spell, including through wands, scrolls, on-hit effects, and items, so these items may prove far more useful in the hands of an accomplished mage.