Spell Reflection is a warrior self-buff that allows the caster to reflect a spell back onto the caster. It lasts for 5 seconds, and can be used both offensively, and defensively.
Improved Spell Reflection: allows you to reflect the next spell used on the closest 2/4 party members as well, and reduces the chance to be hit by spells by 2/4%.
Some "boss" mobs in general and a lot of mobs in raids are immune to their own spell, but reflecting it means you don't suffer its effects, such as Fel Fireball against Lord Jaraxxus in Trial of the Crusader.
A well-timed use of Spell Reflection can greatly improve the results of an encounter. An example of this is in the Caverns of Timedungeon, The Black Morass, against the mage-type Rift Keepers' Pyroblast. The long cast time should allow a sufficient reaction to reflect this powerful spell.
On heroic and normal instances it's sometimes very useful ability to tank casters. For example, one could activate Spell Reflection to deflect an incoming spell. This not only helps reduce damage taken, but also generates threat.
Something less known about Spell Reflection is its ability to reflect multiple spells if they are both cast while the buff is still active (i.e. leave the caster's hand). This makes it very useful when coupled with a charge, as it allows the warrior to reflect the initial spell from each casting mob simultaneously if their casts have the same length and start at the same time.
As of Patch 2.1, reflected spells generates threat as if the damage were dealt by a normal attack. Also, this threat is subject to the usual threat multipliers granted by Defensive Stance and/or Defiance. This change was not officially noted, but it has been confirmed by several players.
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