The short version is that a Zobel circuit for a voice-coil driver loudspeaker is an RC damper that is formed with a capacitor and resistor connected in series. The values for each component can be calculated using the formulas on page 37 of the π Crossover Document. The damper is connected directly across the woofer, and it removes a resonant peak that will form due to the interaction between the speaker's voice coil and crossover components. You can see an example of this peak on page 47 of the crossover document. Subsequent pages show the improved response curve resulting from inclusion of the damper in the circuit.