A shunt is a device used to pass around a point in a circuit by creating a low resistance path. Shunts also allow for current or voltage to be measured when the current on a DC line is high enough to damage devices directly connected to the line.
The resistive element is placed directly on the DC line in series and provides low enough resistance as to not to impede or disrupt the flow of current, but just enough so that a range can be sampled.
Shunts are also place inline with sensitive equipment like PLCs or current monitoring devices, and provide a specific effect if the current is higher or lower than desired. Current that is too high, for example, can be diverted away from the controller to prevent a short or overload fault.