Block is a mechanic that will prevent damage from a source where you would normally take damage, unless explicitly stated otherwise through a passive or affix. There are certain sources of damage that cannot be blocked. They will be explicitly stated in their respective articles for the monster.
Block is based on a chance. When you would normally take damage and you have a source of block it may activate and prevent the damage.
By default you can only block once every 2 seconds. This can further be enhanced through modifiers or passives.
Block has a hard cap at 40%
Block can be granted through off-hands, such as shields, as a starting point and can be further enhanced through Modifiers or Passives.
If an offhand would provide block chance it would specify the block chance is is providing at the top of the item tooltip.