What does it mean?
Three things are needed for the engine to run: air, fuel, and spark. To run well, the ECM (engine control module) precisely meters how much fuel to inject for the measured mass of air at any given moment, and then deliver a spark to ignite a power stroke, hundreds of times per second. Of course, this all requires precise timing, for which the ECM uses the camshaft and crankshaft position sensors (CMP and CKP). The crankshaft position signal serves two important functions.
First, it functions as a speed sensor, which the ECM uses to calculate engine speed in revolutions per minute (rpm). Second, CKP functions as a position sensor, which the ECU uses to calculate the position of cylinder #1 at top-dead-center (TDC). The ECU uses rpm and TDC measurements to calculate injector pulse width (IPW), ignition timing, and variable valve timing (VVT), among other things, and can even detect cylinder misfires.
If ECU cannot detect enough voltage coming from the CKP sensor, it sets DTC P0387 Crankshaft Position Sensor “B” Circuit Low Input. Sensor B is indicative of the secondary CKP in a system that may utilize multiple crankshaft position sensors for various functions.