King's Shield is a move with +3 priority. It protects the user from any attacking move. For example, King's Shield will protect you from ThunderBolt. But it won't protect you from Confuse Ray or Attract.
Also, King's Shield lowers the Attack sharply of a foe that makes contact while using a physical move. For example, Earthquake is physical, but doesn't make contact, whereas ThunderPunch is physical and makes contact.
Source - Battling my brother's Aegislash over 20 times.