Aliasing is what happens when a signal contains frequencies above the Nyquist limit (half the sample rate) and they are sampled anyway: instead of being captured, they fold back down into the audible range as false, inharmonic tones that were never in the original. Once aliased they cannot be removed — they sit on top of real content. The defence is an anti-aliasing low-pass filter in front of the ADC that strips everything above Nyquist before sampling, and a matching filter after the DAC. Aliasing also appears inside DSP whenever you generate harmonics or resample carelessly, which is why high-quality sample-rate conversion matters.
