A ladder filter is a low pass filter design made of four cascaded stages, with feedback from the output back to the input to create resonance. Robert Moog patented the transistor ladder design in the 1960s, and it gives a 24 dB per octave slope with a thick, rounded character.
At high resonance, the bass level drops, which is part of its sound. Diode ladder filters are a variation known for a different, more aggressive response.