- Used to cast spells. Magicka is naturally restored by resting.
Magicka is the energy used to cast spells. Your total Magicka is determined by multiplying your Intelligence by your Magicka Multiplier, which is 1 by default. Fortify Maximum Magicka effects from Races or Birthsigns can increase this multiplier additively.
The blue Magicka bar in the lower left corner of the screen indicates your character's current and maximum magicka.
Bretons have a +0.5 modifier, and Altmer +1.5. Other races don't have a bonus. The Mage will add a +0.5 modifier, The Apprentice +1.5, The Atronach +2.0. For example, an altmer (+1.5) born under the sign of the Atronach (+2.0) will have a total Magicka score equal to (1 + 1.5 + 2) = 4.5 times their Intelligence.
Every spell has a casting cost, which is dependent on the base cost of the spell's magic effects and the magnitude and duration of these effects. Every time you cast a spell, whether you succeed or fail, the casting cost is deducted from your total Magicka pool. Your chance to successfully cast a spell is calculated based on a number of factors, including the magicka cost of the spell, your skill in the relevant spell school, and your Willpower—see here for the complete formula.
You can replenish your Magicka by using potions or scrolls with Restore Magicka, or by resting. You will replenish 0.15 × Intelligence points of Magicka per hour. Alternatively, you can use Spell Absorption effects, which will absorb power from incoming spells (either beneficial or harmful) and replenish your Magicka instead. Finally, you can use Absorb Magicka effects to steal Magicka points from your opponents (which has the added benefit of limiting their spellcasting ability as well).
Those born under The Atronach birthsign do not gain Magicka by resting.