Downsampling converts audio from a higher sample rate (e.g., 96 kHz) to a lower one (44.1 kHz for CD). The signal must first pass through a low-pass filter removing frequencies above the new Nyquist limit (22.05 kHz for 44.1 kHz). Without this filter, those high frequencies alias into the audible range.
Downsampling is done to reduce file size or match target format requirements. Modern sample rate converters use steep, high-quality filters that are audibly transparent. Avoid multiple round-trips of sample rate conversion — each pass slightly degrades the signal.