Sensored/sensorless:

Brushless motors use 3 electromagnets in the stator. In order to turn the rotor, they have to be powered in a certain pattern/speed, so that the magnetic fields push/pull the rotor just right.

A sensorless system has no information about the position of the rotor to the electromagnets at 0 RPM, so the ESC cannot send the correct pattern to the motor, but uses a generic one. As RPM builds up, a kind of feedback signal builds up in the motor wires that the ESC can "read", so with higher RPM it works better and better and the pattern gets more and more efficient.

Sensored motors have hall effect sensors that know the exact position of the rotor all the time and use an extra cable to pass that information to the ESC, that then can send the correct power pattern regardless of RPM - even from standing still.