Hokay... so a voice coil... is a coil of wire. (starting at the basic)
Wire has resistance... like a resistor.
When you put 2 resistors "in series", the resistance adds.
2 ohms + 2 ohms = 4 ohms , u dig?
That is how each sub is wired, "in series". Signal is sent to the +'s (top-left), and travels through the coils to the -'s (top-right). Then, is connected in series to the second coil (DVC sub)'s + (bottom-right). The signal passes through the second voice coil, getting and has a total resistance of 4 ohms from top-left to bottom-left.
Parallel is the opposite. When the two subs are wired in parallel, they have to "split" the signal. So when you have 2 - 4 ohm loads in parallel, it acts like 1/2 the load, or 2 ohms.
It's similar to water piping, if you have 2 pipes flowing parallel, it presents 1/2 of the resistance (twice as much flow).
Things get more complicated past this, but for now keep it simple.
