FFT is usually calculated for a fixed length of time record. When the signal is discontinues, the abrupt ending on both ends of the signal causes a phenomenon called 'spectral leakage '. When a signal is terminated abruptly and analysed using FFT the spectrum spreads into all the the FFT bins rather than confining into a particular FFT bin. The spectral leakage can be reduced considerably if the signal amplitude can be gradually reduced to 0 at both the ends rather than abruptly cutting the signal. This type of technique is called 'windowing'. There are different types of windowing function are generally used, the most common one is 'hanning' window.
Fig 14.0 The software supports different windowing functions for FFT . The most popular windowing function is 'hanning'.