I tried to simulate the input impudance of a multipass feedback low pass filter. I want to know if the graph I got is right, and if so, what does it show? Also, what is that dashed line about?
And here is the circuit itself:
Yes, you have plotted the input impedance of your circuit.
The solid line is the magnitude, and the dashed line is the phase (relative to the right-hand scale) of the impedance. The phase might be 180 degrees off due to the definition of I(V1). If I(V1) is the current going in to the positive terminal of the supply you should use -I(V1) to get the current going out into the circuit.
Having the magnitude on a dB scale is not especially helpful -- better to figure out how to plot it on a linear scale. But this is not necessarily easy in LTSpice, it took me a while to figure out how to do it when I was doing something similar the other day, and I can't remember the correct incantations now.
If you look at the graph you can see that in the x-axis there are frequency values, and in the left and right y-axes, respectively, there are gain and phase values; so basically what you see is the representation in the frequency domain of the input impedance of your circuit.
The problem is that, plotting the ratio between a voltage and a current, in which the latter is in the sub-mA range (given the resistances), you end with a minimum gain of 60dB (=1000).
I'm not sure about what you expect to see, but you can see this as a transfer function that relates the input voltage to the input current. If you want to see a lowpass function you would better look at the transfer function (Vout/Vin).
As said in the previous answers, this is the impedance indeed. The impedance has a magnitude and a phase. This enables one to represent impedances as complex numbers and their values are usually frequency-dependent, hence the representation in frequency domain.
The plots you have are so-called Bode plots. These characterize the amplification (or gain) and phase shift in the response of a system. So basically the magnitude plot (continuous line) is the ratio of the peak values of the output and input and the phase shift is the delay of the output compared to the input.
The Bode plot shows to what extent a filter can suppress different frequencies and whether it is going to introduce any delay.
As a closing remark, Bode plots are used for studying other systems than filters, too. They can be used to determine the stable operation regions of a given system.