The FRC Programming Guide found
here has information about the functions of the WPILib VIs.
The GetSpeed VI outputs the -1.0 to 1.0 scaled value of the PWM output being fed to the Jaguar. This would be the equivalent to getting the 0 to 255 value of the PWM output in the old control system. This value is not necessarily directly related to the motor speed as there are other factors such as the load on the motor and motor's torque/speed curve.