Bass management determines which speakers handle which frequencies. In a 2.1 system, frequencies below the crossover point (typically 80 Hz) are sent to the subwoofer, while everything above goes to the main speakers. Proper bass management requires correct crossover frequency, slope, phase alignment, and level matching.
Without bass management, the main speakers and subwoofer can overlap in frequency — causing peaks and nulls as the two sources interfere. Most AV receivers and some studio monitor systems include automatic bass management with room correction.