What is this

That circuit models a high frequency transmission line. It is important that the load, in your case the 50k resistor, matches the impedance of the transmission line, typically it's 50 ohms. Look into what happens inside a transmission line at high frequencies - it's quiet impressive.

Transmission line model. A coax cable (think of the cable to a modem or a radio), has distributions of capacitance between the inner conductor and outer shell, and inductance along the cable length. It is modeled by the Telegraphers Equations, which is a one-dimensional solution of Maxwell's Equations (derive it yourself, it's fun!). Anyway, in the original version, the line has a 50Ohm load, which is matched to the transmission line's impedance. In the plot of that, you see the gradual progression of energy from one set of reactive elements all the way down the line (pop quiz: how fast does this electromagnetic energy to propagate to the load?). In YOUR case, your load is enormous! I can delete that component and the plot has not changed. So what IS going on in that plot? Think about what happens if your coax cable is just unterminated on the bench. The energy gets reflected! If it's short, the same thing happens (energy returns to you), but the phase information is different. Its not like the coax end will radiate a ton of energy into the air just being open! If it did, we wouldn't bother with fancy, delicate antennas!