Shield Wall is a warrior ability learned at level 48 for those with the Protection specialization. It is a defensive self-buff that costs no Rage. It has a three minute cooldown. When activated it absorbs 40% of all incoming damage for eight seconds.
Shield Wall falls into Panic button territory, and is best saved for when you just have to survive a little bit longer. Examples include an OOM healer, just a few adds too many, an enraged end-instance/Raid Boss, too many stacks of a debuff that reduces healing received, or a Battle Ground flag run.
Shield Wall is the one of the most powerful form of damage reduction in the game, and is extremely effective at keeping the warrior alive. In addition, shield wall does not drop aggro, so it allows a warrior to continue tanking, unlike Blessing of Protection.
For more day-to-day panic, there may be less extreme measures available: [Hamstring]ing or using the much faster cooldown [Shield Block] while you finish it off, or even just dying and being raised again by the group healer if it doesn't look like a total wipe is hinged on your survival. Bear in mind that it isn't an invulnerability shield, and even 40% damage from a ?? Elite can still hurt plenty.