Distortion occurs in a composite signal

1) What is meant by "Distortion occurs in a composite signal " ?
2) Explain this "we can add a new line each time a new channel is needed" ? 
3) there is any disadvantage of sending empty time slots ?

Distortion is usually unwanted. Any change in original signal that alters the basic waveform (shape) or the relationship between various frequency components; it is usually a degradation of the signal.

Your other point is not clear. Make it concise and from which topic you are extracting it.

An empty time slot in Time division Multiplexing techniques is wastage of channel utilization.