systems, the bass roll off describes the way that output diminishes for very low notes.
My own bass speakers have their -3dB point at 40Hz, and are a lowish Q alignment, with a 10dB / octave bass roll off.
This means that if they were playing white noise, with the output at 100Hz being a pretty loud 90dB, the output at 40Hz would be 87dB. The output an octave below that (at 20Hz, a halving of frequency) - would be 77dB, that is, 10dB lower than the output at 40Hz.
Different designs are characterised by different slopes to their bass roll off. Acoustic suspension and transmission line enclosures have a shallow slope, but other types, notably bass reflex (ported) enclosures, have a very steep slope below the -3dB point. Depending on the type, their bass roll off is between 18 and 30 dB per octave.