Actuator Enhancement System. It's new tech that originated in Tactical Operations. Stuck in a mech's arm, it gives a -1 bonus to all attacks made with that arm (applying to all arm-mounted weapons as well as physical attacks)- it's not compatible with a Targeting Computer but it applies to weapons that TCs can't link to, like missile pods. In a mech's legs, it gives a -1 bonus to Piloting rolls.
And to clarify StCptMara's post, the new Urbanmech variant mounts a total of three SSRM-6s- two in the right arm and one in the left, and also packs AES in both arms.