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: The enemy used Move and Attack, AND They chose not attack. 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.
I have an Apprentice Wizard on site D. You have an untapped Bosk Troll on site C, and both Queen of Midland and Queen Guinevere on site B. I would love to cast my Poison Nova to kill all your minions, but I need to get my Apprentice Wizard to site C first. If I use the Move and Attack ability to move my Apprentice Wizard to site C and then attack, your Bosk Troll will be able to defend. If I move to site C but don't attack, your Bosk Troll will be able to intercept. Either way, my Apprentice Wizard would die before being able to cast the Poison Nova on site C. In the end, I will probably not choose to move to site C, and instead do something else. Intercept is a deterrent to certain basic movement, even if you don't actually use the intercept ability during a game.
You have Rimland Nomads on site D. I have an untapped Bosk Troll on site C. You tap your Nomads, using the Move and Attack ability. As the ability resolves, you declare the following movement path: Surface site D -> Surface site C Surface site C -> Surface site A Rimland Nomads have Movement +1, so you resolve that movement normally and they're now on the surface of site A, having moved past the Bosk Troll without any problem. On the surface of site A, they choose not to attack. (If they did attack, my Bosk Troll could defend, killing them.) Even though your Rimland Nomads didn't attack, my Bosk Troll cannot intercept since it doesn't share the final location with your Rimland Nomads.
You have a Foot Soldier token on site C. I have an untapped Bosk Troll on site C as well. You tap your Foot Soldier, using the Move and Attack ability. As the ability resolves, you declare the following movement path: Surface site C -> Surface site A You resolve that movement normally and they're now on the surface of site A, having moved away from the Bosk Troll without any problem. On the surface of site A, they choose not to attack. (If they did attack, my Bosk Troll could defend, killing them.) Even though your Foot Soldier departed a location shared with my Bosk Troll and then didn't attack, my Bosk Troll cannot intercept now since it doesn't share the final location (surface of site A) with your Foot Soldier. Remember, the two things that are relevant for intercept are: Did the enemy use the Move and Attack ability, AND Did they not attack? At the moment they choose not to attack, that's the moment you can tap to intercept, at that location.
You have an Apprentice Wizard on site D. I have an untapped Bosk Troll on site C. You cast Blink to teleport your Wizard to site C. My Bosk Troll cannot intercept since you didn't Move and Attack. I can only intercept if you use Move and Attack, then don't attack.
Units can technically tap to use their Move and Attack ability without moving and without attacking since both are optional, perhaps anticipating Mesmerism. If the unit does not attack, you may intercept! For example, you have untapped Amazon Warriors and I have an Autumn Unicorn at the same location. If I tap my Autumn Unicorn to use its Move and Attack basic ability, then decline to move and decline to attack, your Amazon Warriors may tap to Intercept.