What does 6kV , 5MW 90% efficiency at full load with 0.9 Power Factor lagging represent for a motor?
5MW is the mechanical power the motor shaft puts out.

6kV is the source line voltage in to the motor.  What the motor needs to run at rated operation.

To get 5MW mechanical power out of the motor at an efficiency of 90%, means that 5MW + 10% electrical power must be put in (5.56MW).

$$ \eta = \frac {P_{out}}{P_{in}} \times 100%$$

Finally, the power factor (a little bit more complex).

The motor consists of a stator and rotor.  The stator has windings that create a magnetic field to cause the rotor to rotate. These windings are an inductive load \$X_L\$ and they consume reactive power \$Q_L\$.  So the source has to put in real power \$P_{in}\$ (MW) to drive the mechanical load and reactive power \$Q_L\$ (MVAR) to cause the motor to spin.  This power is called apparent power, S (MVA).  

The power factor is a sort of efficiency of the motor/circuit.  Power out of motor vs what the source has to generate.  

$$ pf = \frac {P_{in}}{S_{in}}$$

To get 5MW at 90% efficiency and 0.9 lagging power factor, the 6kV source supplies 6.17MVA of apparent power at current of 660A.