"Block" is a passive defense which prevents an opponent from landing a blow in combat.
It depends on a player's bonus in fighting.defence.blocking and controlled by the option option combat tactics settings. Holding a shield, along with the qualities of the shield and the skills of the attacker all factor into the success of a block.
In addition to blocking attacks, the player also gains the absorb mechanic (or rather, holding a shield protects the user on an unknown level and acts like armour with unknown coverage, absorbs can be quite powerful but are vastly unknown as to what causes the ability to trigger.) Absorbs cost little to no ap and basically just add to the defense of the character.
Block has yet to be tested extensively, or the poeple that have done the testing haven't shared it here.
(Check "discussion" for notes from users about block)