The phase switch affects the soundwave the subwoofer produces -- when you flip the switch, the sound wave becomes inverted. For example, instead of moving in out in out, the speaker come will move out in out in.
The switch is useful to help the sub blend well with your mains. If you think your sub isn't blending well with your main speakers, try flipping the switch to see if it helps.
Chances are you have the sub connected to a receiver that handles the crossover, so the phase switch might not have a noticeable effect.
I hope I explained things at least semi-clearly.