A: In this case, “series” and “parallel” refer to how the two coils in the humbucking pickup are connected together. With series wiring, the output of one coil feeds the input of the second coil. This is the “traditional” humbucker we’re all accustomed to, with its thick, full, rich tone.
When the two coils are connected in parallel, the two coils’ outputs are tied together and the inputs are tied together. This results in a tone that is somewhat similar to a single-coil, yet it still retains its humbucking characteristics, so noise isn’t a problem.