Trailer brakes? What does electric over hydraulic mean?

Electric - The voltage output from the controller determines how much braking effort the magnets exert on the wheels.

E/H - The voltage output from the controller determines how much hydraulic pressure the actuator (pump) generates. The hydraulic pressure determines how much braking effort the disk brake pads exert on the wheels. Control is more precise, which is why auto manufacturers went to disk brakes.

Note that it would be possible to use E/H system with hydraulic drum brakes, but I've never seen it.