Transient response of a transmission line

I want to know dat what is meant by transient response of a transmission line.
I have searched different sites and i am unable to find what i want.. my exams are about to come and this kind of ques has been asked repeatedly...here is an example..

*A lossless line of long length having characteristic impedance Zo= 50 ohm, load impedance = 250 ohm is exited by a source of 20V, 100 ohm. Find the transient response of the line.*

thanks in advance..