What dose this result mean?S12?

Dear friends
I have simulated the coax line with ADS and got the following result,but I don't understand what dose really this result means?

would you help me,please?

with the log scale in X axis the above result is:

You are looking at the insertion loss through the sample length of coaxial line as a function of frequency. These graphs simply show that the loss per unit length of this transmission line increases as the frequency is increased.

Dear RealAEL
Thanks for your reply.you are right, but I want to know the above graph meaning in terms of the delivered power.
so, what dose this graph say us about the delivered power to the load?

I suggest S21 = delivered power - source power (each in logarithmic units)