The "signal" is the voltage level versus time between the center conductor and the ground potential at the destination. In an ideal cable this potential (voltage) is the same at the source and destination.
In an ideal termination, any current flowing in the center conductor will be in perfect phase with the voltage.
In "real" cables there are slight out of phase current flows due to capacitance, induction and dissapative dielectric losses in the non-conducting materials. This will cause slight variation between the voltage at source and destination. In other words, cables will have a transfer function.
Cable designers should strive to make the cable have a transfer function which is equal to 1 (ideal cable). However a non-ideal cable can have transfer functions that make it sound pleasing. These like other elements in our systems are really distortions. But not all distortions sound bad/
