What actually Power factor is, in practically?
The practical uses of the power factor vary, but its primary physical manifestation is the quotient of active current to total current flow in the conductor. There is actually more current flowing in the conductor if the power factor of an AC device is less than 1.

This is useful for instance for Joule loss calculations; if you know the power factor of a device is 0.5 and its active current draw is 1A, you know that you can model the losses in the conductor as 4 times the losses that would occur if the power factor were 1:

$$P_{loss} = I_{apparent}^2R = \left(\frac{I_{active}}{PF}\right)^2R$$

Another application is for the calculation of required wire gauge; if a device with a particularly low power factor is supposed to be connected to a wire, care must be taken to oversize the wire to accomodate the extra current that will flow.