What does it mean?
In vehicles with electronically-controlled transmission shifting, a module is used to control when the transmission shifts and which gear should be engaged. The module modifies voltage to individual shift control solenoids which in turn direct fluid pressure to the proper channels according to the shift lever position and the speed. If DTC U0101 is present, there has been a loss of communication with the Transmission Control Module (TCM), either current or history.
It can be a fault between two modules or a communication failure on the CANbus line. Because the CANbus line is used for two-way communication, a disruption can mean that solenoid positions are not known and the transmission will resort to default positions. Often, when a fault occurs that sets DTC U0101 and subsequently clears up, cycling the ignition may restore the vehicle's normal function.