Ambush is a unit ability that allows the unit to defend itself first and, if this was the first attack on this unit during the round, receives no damage if the attacking unit is destroyed by this.
How it works
If a unit with Ambush ability is attacked for the first time during a round, then the Ambush unit will automatically deal the damage first and, if the attacking unit is being destroyed by this, then the Ambush unit will receive no damage.
If the unit has been attacked already this round and used its Ambush ability this round, then the Ambush mechanic will not trigger and the attack will be conducted regularly.
How to identify Ambush units
Ambush units can be identified by the little crosshatch below the unit on the battlefield.
Alternatively, if you look at the card you will see the word "Ambush" written beneath its image.
The story behind Ambush
Ambush is a warfare tactic that has been applied throughout the ages. Implementing this tactic into KARDS adds an element of depth and tactic to influence the battle.
Once an Ambush was successful, obviously the unit is out in the open and your opponent knows about it. As such, using an Ambush in the same round by the same unit wouldn't make sense. To use the Ambush ability again, you need to wait until next round.