The processor setting that redirects low frequencies from your speakers to the subwoofer, governed by a crossover frequency.
Most speakers — especially compact ones — can't produce deep bass cleanly, and trying makes them distort. Bass management fixes this by setting each speaker to "small" with a crossover (commonly 80Hz), sending everything below that to the subwoofer, which is built for the job. The dedicated low-frequency effects (the ".1") channel goes there too.
Getting the crossover and subwoofer level right — ideally with room correction — is one of the highest-impact, lowest-cost improvements you can make to a home cinema. Running big speakers "full range" and hoping is the classic mistake.