Subtractive synthesis
Start with a bright, buzzy tone and carve it down with a filter. The classic analog recipe.
Subtractive synthesis works backwards: instead of building a sound up, you start with a harmonically rich tone — usually a buzzy sawtooth — and then take pieces away with a filter until only the sound you want is left. It is the classic analog recipe, and the sound of every acid line.
The star of the show is the filter. A lowpass filter lets low frequencies through and turns down everything above its cutoff frequency. Slide the cutoff down and the sawtooth above goes from bright and buzzy to dark and muffled — you are literally removing its higher harmonics.
Resonance
Resonance boosts the frequencies right around the cutoff, adding a whistling emphasis — the peak you can see and hear in the demo. Push it far enough and the filter starts to ring on its own. Sweep the cutoff with high resonance and you get that vocal, squelchy wow that defines acid.
Point an envelope at the cutoff instead of your hand and the filter opens and closes automatically with every note. That is subtractive synthesis in a nutshell: rich source, filter, envelope.