Energy shield is a property that appears on armour with an intelligence attribute. It can be modified by that item's quality and magical affixes. An energy shield absorbs damage that would normally reduce a character's life total. The character will not lose any life from damage until the energy shield is depleted. Energy shields regenerate while the character is out of combat.
The shield appears on the user interface as a light blue bar just above the life orb in the lower left of the screen.
Further details of how energy shields work are currently unknown.