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When you sample a signal and you take the samples to perform the FFT, you dont take the whole signal, usually you take what is called a window interval of the signal. The signal samples in the window interval can or cannot have interger number of cicles within it, if it doenst have a integer number of cicles spectral leakage will occur.
The interval between each sample is related with the frequency resolution, if this interval is a submultiple of the signal period it is possible to window a integer number of cicles, if it's not it is impossible to window a integer number of cicle. Anyway if in the window interval there are not an integer number of cicle the signal analysed is not itself, as a result side lobes will appear around the real frequency . Obviously it is a distortion, this phenomenum is known as spectral leakage.