Roll-off, or slope, describes how quickly a filter reduces frequencies past its cutoff. It is measured in decibels per octave. A 6 dB per octave filter is gentle, 12 dB per octave is more pronounced, and 24 dB per octave is steep, as in a classic Moog ladder filter.
Steeper slopes make a filter sound more decisive, and gentler slopes leave more of the original sound.