Current measurement allows for overcurrent protection (stalling) as well as feedback to detect and compensate weather helm and battery fluctuations.
The basic controller uses an arduino either directly driving an h-bridge with mosfets, or a drone, or rc style ESC motor controller. These brushless controllers must be reprogrammed to drive a brushed motor.
In the above controller, the white wires supply power. The Red and Black wires go to the drive motor which moves the rudder. The four smaller wires are attached to the serial port of the autopilot computer running pypilot.
This controller can supply up to 3 amps continuous, or 20 amps in bursts allowing it to detect stalling without issues.