When hit by a spell, you have a X% chance to boost resistance to that school of magic for 18 sec. Stacks up to 3 times.
Acclimation is a Death knight talent located in the 9th tier of the Frost Tree. Each stack increases the resistance against one particular school of magic by 50, 3 stacks will therefore increase magic resistance by 150. It is possible to have full stacks of acclimation against several schools of magic at once. Acclimation stacks with other sources of magic resistance, such as Shadow Protection and Frost Resistance Aura.
The 18 seconds of duration are not enough to keep the buff stacked during a fight in a predictable manner if only one enemy is attacking the death knight. The talent is most useful in battle grounds, for AoE tanking and against channeled spells.