Spell Reflection is a level 47 warrior ability that reduces magic damage taken and allows the warrior to reflect a spell back onto the caster. It lasts for 5 seconds or until a spell is reflected, and can be used both offensively, and defensively.
Despite the nature of the ability, with patch 5.4.0 Spell Reflection no longer requires a shield to be equipped. If the warrior does not have a shield equipped, a visual of an equipped shield will be displayed.
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.
On heroic and normal instances Spell Reflection can be a very useful ability when tanking casters. This not only helps reduce damage taken, but also generates threat from the reflected spells.
Something less known about Spell Reflection is its ability to reflect multiple spells if they are both complete casting while the buff is still active. 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.
Patch 9.0.1(2020-10-13): Available again as a baseline ability for Arms and Fury. No longer requires a shield.
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.