A low-pass filter (LPF) cuts everything above a set cutoff frequency. The slope (e.g., 12 dB/octave, 24 dB/octave) determines how sharply frequencies are attenuated above the cutoff. In a subwoofer crossover, the LPF ensures the sub only reproduces bass (typically below 80–120 Hz).
LPFs are also used: as anti-aliasing filters before AD conversion (removing frequencies above the Nyquist limit), to darken bright recordings, and in synthesis to shape oscillator timbre. Every parametric EQ includes high-pass and low-pass filter options.