Most filters don’t completely suppress the portion of the signal that falls outside the frequency range defined by the Cutoff parameter. Frequencies that are located close to the cutoff frequency are generally reduced less than those that are farther away. The higher the slope value, the more apparent the level difference is between frequencies that are near to the Cutoff frequency and those that are farther away from it.
In Logic Pro, click the 12 dB or 24 dB button. The slope (curve) chosen for the Filter expresses the amount of rejection in decibels per octave. The steeper the slope, the more severely the level of signals below the cutoff frequency is affected in each octave.
Logic Pro Instruments: PDF