Defend is a basic ability that allows you to defend against your opponent's attacks. A unit must be untapped to defend, and it's one of two basic abilities that you can use on your opponent’s turn (the other is Intercept). Defend is a triggered ability with the following text:
When an enemy attacks another unit or site in this unit's range of motion, it may tap to defend, moving to the attack's location and then joining the fight.
When an attack is occurring, there is a window to defend. See the "attack" codex entry for further details. During that window, multiple different units can defend, one at a time.
When a unit moves to defend, it may use any of its movement abilities, such as Airborne, Burrowing, Movement +X, Submerge, or Voidwalk.
After completing its movement, assuming the unit successfully reaches the attack location it will then complete the Defend action, changing the target of the attack to itself.
If the original target of the attack was a unit, the controller of the unit using the Defend ability decides if the original target remains in the fight, or is removed. If the original target was a site, it is automatically removed.
Remember, only “attacks” can be defended; attacks are only caused by the Move and Attack basic ability, or very specific card text that uses the word “attack," e.g. .
When a unit uses its Defend ability, it automatically becomes a legal target for the attack as long as it completes the Defend action successfully. For example, a minion with Stealth normally can't be targeted by an attack, but if it chooses to Defend, it will be a legal target (as an explicit exception to the normal precedence in the Silver Rule).