How is the sinc function used to reconstruct an analog signal from a digital signal?
To elaborate on Samuel's comment. When you sample a signal the spectrum of that signal will become periodic in the Nyquist frequency. To reconstruct the actual signal, you need to remove all of the frequencies that appear as artifacts of the sampling process. Ideally, this would require a brick-wall low pass filter. In the time domain, this corresponds to the procedure your lecturer mentioned with sinc functions, as sinc is the inverse fourier transform of the rect function.