Your spell criticals will refund 15/30% of their base mana cost.
Master of Elements is a popular PvE talent for any talent build while spending points in the Fire tree. It restores up to 15/30% of the mana cost of any spell that critically hits one or more targets (so AoE spells such as Flamestrike, Cone of Cold and Blast Wave are affected by this talent). Since most mages critically hit between 20 and 40% of the time this talent is equivalent to an effective mana cost reduction of all spells which can critically strike.
Other talents that reduce mana cost include Arcane Focus, which reduces mana cost of Arcane spells up to 3% directly, Precision, its Fire and Frost equivalent, Arcane Concentration which works out to about a 10% reduction (on all spell types), and Frost Channeling which directly reduces the mana cost of all spells by up to 10% (depending on talent rank). In addition Pyromaniac also reduces the mana cost and increases the critical strike chance of all Fire spells by 1%/2%/3%.
For channeled spells that deal damage in ticks such as Arcane Missiles and Blizzard, the mana returned is based on the tick that critically hit, and not the entire spell. For example, Arcane Missiles in its higher levels deals damage in 5 ticks: it will only return 15/30% of 1/5th of the cost of Arcane Missiles and not its total. However, because these two spells have high mana costs, Master of Elements can still go a long way in conserving mana for a mage who uses either.
Patch 3.0.2 changed this talent to affect all spells, not just Fire and Frost ones.