phenomenon when window size for STFT is too short
It might mean that if you take a window of not enough length so as to contain at least one period of a sinusoid, and instead you are only getting a lot less inside the window of analysis (Like 10% of the period or something like that), then the fourier transform will show you the spectrum of a signal as if that little piece you got inside the window, extended periodically was the entire signal.

To know exactly what is going on in your graph, we need to know the scale of the time axis, and which signal was being analyzed, if it was a sinusoid, what was the frequency. but that might be what the book is trying to say.