AC signal to a coaxial cable
first you have to understand that R, C and L are electric parameters that can be built up from special structures and materials. For example, a capacitor is a couple of paralel plates isolated with a dielectric, and its capacitance is determined from plate's area (A) and separation (d), kind of isolation (E), so C=EA/d, in general from material and geometry. In this idea one can divide any transmission medium like a coax into several segments, a very short segment whose length tends towards zero, if you look carefully to one segment you will find out that its structure in esence is quite similar to that of a capacitor, plates no planed but curved, and dielectric, so that short segment should have capacitance too. Similarly, one can say the same for the other electric parameters: resistance and inductance, and deduce that what has in a cable is really a AC circuit with RLC components.

Well, the RLC parameters in a cable are significant only if the transmitted signal through it has a wavelength comparable to the cable length, otherwise one can just look down on them, and this occurs generally at low frequencies in communication applications. So when you are working a low frequencies and need RLC parameter you have to work with resistor, inductor, and capacitor components (lumped elements) but at high frequencies you just need to consider only the RLC component in the line itself (distributed elements), and some times you can use a hybrid approach: lumped and distributed elements in order to built a AC circuit with RLC.

Hope you can understand the concept.