The low-pass (lp in Reason) filter is a filter that passes low frequencies well, but reduces frequencies higher than a certain "cutoff" frequency. The cutoff frequency can be adjusted with the slider, allowing you to choose the amount of high frequencies to cut away.

Besides cutting away the frequencies higher than the cut-off point, one can also determine to boost the frequencies near this cut-off frequency. The amount of this boost is dertermined by the resonance setting of the filter. If you raise this one too much, you'll find some frequencies boosted so hard it hurts your ears!