Intercept is a basic ability that allows you to fight your opponent's units who "Move and Attack" without actually attacking. A unit must be untapped to intercept, and it's one of two basic abilities that you can use on your opponent’s turn (the other is Defend). Intercept is a triggered ability with the following text:
When an enemy using its Move and Attack ability finishes its movement and does not attack, any of your units there may tap to intercept and fight that enemy.
Units cannot move when they intercept; they must already be at the enemy's final location.
The timing for an attack and the timing for intercept are the same; if the enemy attacks, you may defend. If the enemy doesn't attack, you may intercept. In summary, if both of the following are true, you can intercept:
Units with Stealth cannot be intercepted, and units with Airborne can only be intercepted by enemies with Airborne or Ranged.
If multiple allied units intercept the same enemy, they all fight that enemy together.
Only the enemy's final location is relevant for interception. See the examples below for more details.