to STAV:
if you have two coils in your bass you can wire them in a couple of ways, the most used are: series and parallel, both are humbucking
Series are tend to have more MIDs, while Parallel are to have 50% lower output (in compare with series), but more LOWs and HIs.
Every coil has two output wires, parallel does mean that two outputs wired to ground and another pair wired to hot output, wiring in series does mean that you wire one coil output to ground, anether output wired to next coil, and the other output wired to hot output, so it is like two coild are wired next to each other.
I know it is a bad explanation

so here is wiki explanation on the question
http://en.wikipedia.org/wiki/Series_...allel_circuits