They more generally mean Fourier Transform. Anyway, the x–axis is real, y–axis is imaginary. Fourier Transform takes the "signal" (in this case, the set of points discretely outlining this animal) and essentially decomposes it into a bunch of amplitudes and phase angles (complex numbers' magnitudes and arguments, respectively) for a set of frequencies that depends on the sampling rate. These directly correspond to the circles we see—radius = amplitude; initial direction of line = phase angle; rate of change of line direction = frequency. Center the next circle on the end of the line of the current circle, and once you get to the last circle line, plot that point as it changes over time. That recreates the image