Ice shield is one kind of buff that can prevent any kind of damage done to the affected target except piercing type damage.
It acts in every way like additional Hit Points, including its own gauge like a lifebar, except that it cannot be healed back, and the bar shown in blue instead of red. Ice shield points are removed first, and when they are gone, the ice shield bar is removed.
While enemy Ice Shields typically last indefinitely or a long time, those on player-controlled units are typically limited to short duration, or disappear once the unit strays out of a certain range of the caster.
Buildings can also have Ice Shields; Lost Disruptor can cast them on itself, the and the Ice Shield Tower can cast shields on friendlies. The Blessed Affinity version of Hammerfall periodically casts them on units in range