A frequency component of noise or signal at one frequency will contribute to the measured value at an FFT bin . The amount of the contribution is proportional to the amplitude of the window function 's kernel , centred at the FFT bin , measured at the interfering frequency. This amount is what is meant when we talk about spectral leakage .
Spectral leakage changes both the amplitude and position of a frequency measurement.
The effects of spectral leakage are worst when detecting small signals in the presence of nearby large signals.