All characters start with a new attribute called block. Block is calculated as follows:
dexterity modifier(minimum of 0) + block from armor(min 0) + block from shield(min 0) + any temporary block
When a character is attacked by any attack(anything that requires the target to roll a d20 to hit you and deals damage) they can spend 1 instance of block to negate the damage entirely. This is called a block action and is a free action to perform. In terms of game events the character either directly blocks, dodges or avoids the damage(Up to the GM and player to decide how this occurs in terms of storytelling.) Once spent, block can only be regained after finishing a rest.
Certain armor provides block at the start of combat. This block is removed last, and effectively means this: armor increases the amount of block you have after a rest, and if you ever have less block than the stated amount when rolling initiative, then you start combat with the stated amount of block.
Some spells, items, feats, or abilities may grant temporary block. Temporary block, similar to temporary health, is spent before any other block, and effectively functions identical to regular block. Temporary block cannot stack, so when you a spell, item, or effect provides you with temporary block, you either retain the amount you had, or take the new amount if it is higher. The duration of temporary block is 1 hour and is refreshed if reapplied.
After an attacker successfully hits a target creature with an attack, but before the damage is dealt, you can declare that you perform a block action(Or that you simply block, for the sake of simplicity,) in which case you must roll a dexterity check against the block DC of the attack(This is called a block check or roll in other places.) On a success, the damage is negated. On a failure, the damage is taken as usual and the instance of block is lost regardless.
Typically every attack has an associated block DC which is calculated as follows: 8 + proficiency bonus + attack modifier
It is important to note the following:
Items, spells, and abilities can influence the block DC of an attack.
A character that is wearing armor or a shield of any kind can add their proficiency to the block roll so long as they are proficient with that equipment.
A character that is wearing medium or heavy armor, or is wielding a heavy shield can use strength in place of dexterity for the block check.
Some items abilities, spells, and abilities can influence the nature of how a block roll is made further.
A critical hit always incurs disadvantage on any block check made to negate it.
A +1 or similar weapon will increase the weapons block DC by 1.
Some attacks and abilities(and most spells) will specify that the damage cannot be blocked. Un-blockable damage cannot be reduced by the block action. If an attack deals regular and un-blockable damage, then only the regular damage can be blocked, but even on a success, the rest will be applied.
Weapons have been regrouped. Instead of most classes flat out getting access to the entirety of martial or simple weapons, weapons have now been sub-categorized into weapon groups(such as daggers and knives, light blades, crossbows, etc.) which characters gain proficiency with.
Weapons also now come with innate maneuvers, which can be performed by expending an additional bonus action or rest dice during an attack.
The new conditions are still a work in progress, but the list can be found here: