The document discusses periodic sampling of continuous-time signals to create discrete-time signals. It explains that when sampling a continuous-time signal x(t) at intervals of T seconds, the discrete-time signal x(n) equals x(nT). It also states that for the discrete-time signal x(n) to uniquely represent the original continuous-time signal x(t), the sampling frequency must be greater than twice the maximum frequency of x(t), which is known as the Nyquist rate.