what does spell penetration mean?

[Ryu]

I know mages can learn a skill that incresses it but i'm not sure what it does?

[TheGreenGuy]

It reduces the the elemental resistances when calculating damage. On the Character Page you can go to the Penetration Stat pages and hover over the stat to see the tooltip. It does not reduce resistances below 0.

So if you throw a fireball at an enemy with 10 fire resistance, while having 20 Spell penetration, it's like the enemy has 0 fire resistance.