We have already noted the trade-off between time- and frequency-resolution using the DFT. Larger DFT sizes provide better spectral resolution but less time-resolution and vice-versa.
It turns out that we can add zeros to the end of a signal in order to achieve a longer DFT length without modifying the spectral content of the signal. Because the zero-padded signal is longer (though no new energy has been added), the resulting DFT provides better frequency resolution.
For example, a length N=1024 DFT provides a frequency resolution of 43 Hertz at a sample rate of 44.1 kHz. If we add N more zeros to the signal and perform a length 2048 DFT, we double the frequency resolution (to 21.5 Hz).
Stated another way, zero-padding in the time-domain results in interpolation in the frequency-domain.
Figure 1 illustrates the effect of zero padding on the resulting DFT.
A time-domain signal (top), its DFT (middle), and the DFT when zero-padded by a factor of 3 (bottom).