A filter is the most common tone-altering module in a modular synth. It typically reduces the strength of select harmonics or overtones, changing the tonal balance of a sound.
The voltage controlled filter is a major part of subtractive synthesis. There are many different ways to design a filter, and although they basically do the same thing, they all sound a little bit different.
The filter is the part of the synthesizer that is most responsible for shaping the tones you get out of it, and it gives each synth its own character and unique sound.