The Nyquist Frequency is the highest frequency that can be represented by a digital system. It is exactly 1/2 of the sample rate. Frequencies above the Nyquist Frequency will be incorrectly represented in the digital system. Frequencies above the Nyquist Frequency alias (or fold back) into the region below the Nyquist frequency. A frequency 1 kHz above the Nyquist Frequency will fold back to 1 kHz below the Nyquist Frequency. Likewise a frequency 2 kHz above the Nyquist Frequency will fold back to 2 kHz below the Nyquist Frequency.
For example, the CD format uses a sample rate of 44.1 kHz. The Nyquist Frequency is 1/2 of 44.1 kHz or 22.05 kHz. Ideally, any analog input signal above 22.05 kHz must be filtered out before being digitized. If this filtering is not done, frequencies above 22.05 kHz will fold back into frequencies between 0 Hz and 22.05 kHz. A frequency of 23.05 kHz will fold back to 21.05 kHz (22.05-23.05+22.05=21.05).
Oversampling A/D converters sample the audio at very high sample rates to avoid aliasing. The Benchmark ADC1 and ADC16 sample the input audio at frequencies exceeding 6 MHz. These high sample rate digital signals are then filtered in the digital domain to remove frequencies above the target output sample rate. After filtering, the high sample rate digital signal is decimated to the desired output sample rate.