Question 1: Why it is necessary to multiply the signal with HANN window function before fourier transformation?

Question 2: How I may get a hann function value by using pd object?

RFFTs are a compromise between fourier's theories and the need for real time processing. two signals ( a sin and a cos) are passed through the window, and the frequency responses are calculated. Of course, it's not as accurate as sending all possible sin frequencies through your signal and discretely checking each resonance. The HANN window function is one method of increasing the signal response of the FFT. more info on wikipedia :http://en.wikipedia.org/wiki/Window_function.

(this is of course, only my limited understanding of the FFT topic)