Defense Value vs Attack Value
Defense is a value calculated by the total health values of units with the blocker ability that are actively blocking. Note that units stop blocking when they use their ability or they have been chilled equal to their health. Attack value is simply the sum of the attack of the units attacking.
During the Defense phase, the player's defense value is compared to the opponent's attack value. If defense value is greater than the attack value the player will be allowed to assign the incoming damage to their blockers. If the attack value is greater than defense phase, the opponent will proceed to the Breach phase and be able to assign any excess damage to the non-blocking units they choose. In the event defense value and attack value are equal the blocking units will be wiped out and the Breach phase will be triggered but as there is no excess damage to assign, the opponent will simply pass.
As the player, you want to prevent breaches by always having a defense value that is at least one greater than your opponent's attack value. In doing so you can use the absorb mechanic to maximize your defense. For strategical considerations related to defense, see Defense concepts.