Block value is a number typically appearing on shields that sets the amount of damage absorbed when you block an attack. Some abilities, enchantments, items and spells could contribute bonus block value as well. As a stat it appears as "X Block" as the base value on shields, "Increases the block value of your shield by X" as a bonus on shields or a stat on another type of item, and "X additional block value" as an enchant. The block value is increased by one for two points of strength.
Sometimes this number is confused with "block rating", which increases a player's chance to block.