"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 options.combat.tactics settings (dedicated block users attack with one hand and block with the other). 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 people that have done the testing haven't shared it here.
(Check "discussion" for notes from users about block as I'm trying to keep this page clear of theorycrafting, a good idea of block can be gotten from checking out the page on parry and checking the notes in discussion.)