In a lowpass filter, frequencies below the cutoff frequency are low, so they pass through without being filtered. Frequencies above the cutoff are attenuated. The frequency response curve of a classic lowpass filter. No filtering will take place. But if we send the filter a 0dB input at a frequency of 3,200Hz, the output will be lower.
Some synth filters have a selectable rolloff slope, while others have only a fixed slope.
For technical reasons, it's not easy to build a filter that completely stops all frequencies above the cutoff while having no effect on those below. In a synthesizer, the filter is designed to kick in gradually, in such a way that the amount of filtering depends on the frequency of the input. Frequencies near the cutoff are attenuated only slightly. The farther beyond the cutoff a particular sound is, the more it will be attenuated. This idea is illustrated in Figure 1.