An analog signal has only one frequency unless it changes periods.
That is absolutely not true, except for the special case of a sine wave signal. See Fourier series.
Take a square wave, for instance, of period p. Its fundamental frequency (which is what you're thinking of) is f = 1/p, but it also has frequency components called harmonics of decreasing amplitudes at frequencies 3f, 5f, 7f...
The lowpass filter graph shows that the gain (Vout / Vin) decreases as the frequency increases, so if you were to pass a square wave through the filter, the fundamental would remain strong, but the upper harmonics would be reduced in strength; as it turns out this tends to "round off" the corners of a square wave and make it more similar to a sine wave.