Netherwind Presence is a Magetalent in the Arcane tree. It increases the Arcane Mage's passive spell haste, which increases the speed at which he casts spells.
The 3% spell haste increase is roughly equal to having an extra 98 haste rating on one's gear at level 80.
Spell haste is useful for both PvP and PvE as it makes your spells with a cast time such as Arcane Blast quicker and channeled spells such as Arcane Missiles do full damage in less time. In addition, it reduces your global cooldown, making it easier to cast multiple spells in quick succession in PvP or casting faster after an Arcane Barrage in PvE. It does not reduce cooldowns.