A sensorless brushless DC motor (sensorless BLDC motor) is quite simply a brushless DC without hall effect sensors. Hall effect sensors are sensors which are built into sensored motors which are used to tell the controller exactly what the rotor position is. This can be useful for maintaining set speeds and is particularly useful at start up as the rotor position dictates the start up sequence for the motor.
However, sensorless brushless motors remove the sensors and operate without this positional information. This means that the controller has to operate the motor without the use of sensors and must control speed