A trailer ECU is an acronym for Electronic Control Unit.
The ECU is powered by the ISO 7638 connection to the tractor unit and controls both the braking and suspension systems.
These systems include:
EBS - Electronic Braking System
ABS - Anti-lock Braking System
Load-sensing - A system to detect load weight and apply proportionate braking force.
Suspension
RSS - Roll Stability System - an active feature to prevent rollover risk.