The cutoff point of a filter determines the frequency at which the filter starts to reduce the volume of the sound. If, for example, you set a low pass filter's cutoff point to 500 Hertz, the equalizer reduces the volume of all the frequencies above 500 Hertz. Raising the cutoff control lets more high-frequency sound through the EQ; lowering the cutoff control lets less high-frequency sound through, making the audio sound bassier.
The slope of a filter determines how sharply it reduces the volume of the frequencies above the cutoff point. A low slope setting makes the filter gradually reduce the volume, while a high slope setting makes it quickly bring the volume down. If, for example, the cutoff point is set to 500 Hertz, a gentle slope setting would mean that the volume of the 1,000 Hertz point on the frequency spectrum is only somewhat lowered. A steep slope, on the other hand, would reduce the volume of this frequency point drastically. Some equalizers have a dedicated slope control marked in decibels; others use the "Q" control to set the slope.
When you're mixing the different tracks of a song together, use a low pass filter to eliminate unwanted high frequencies from a track. Applying a low pass filter to a screechy violin, for example, can mellow its sound. If a recorded audio track contains static or other high-frequency interference, use a low pass filter to remove it from the recording. You can also apply a low pass filter to the bass or kick drum track to ensure that any high frequencies on these track don't conflict with the rest of the production.
Use the low pass filter on a synthesizer to create bass sounds. Set the cutoff point on the filter fairly low, then increase the slope to add presence and punch. The slope control on a synthesizer's low pass filter is usually marked "Resonance" or "Res." To create a more interesting bass sound, use the synthesizer's envelope or low frequency oscillator to modify the filter's cutoff point, thereby creating a pulsing sense of movement within the sound.