AP = armour piercing; means that armour of a unit is halfed when it is attacked by a unit with "ap" ability. It can be quite devastating, especially on units which mostly rely on their heavy armour rather then defence skills.
Pedites Extr., Samnite Milites, Thracian Peltasts, Thracian Elite Swordmen, Falxmen/Bastarne, most Iberian units ( armed with falcata swords ), Bodyguard Cavalry units, Heavy Cavalry, some Medium Cavalry, etc. etc.
Here you can take a look at all units with their stats: http://europabarbarorum.heimstatt.net/index.php?