What's phase 0 degree - 180 degree?

I have a JM Lab Electra Sub, and it has one switch which marks 0 degree and 180 degree. The switch sets at 0 degree when I brought it. However, the bass is less when I set the switch to 180 degree, and bass increases when at 0 degree. So, my question are
1. What's phase?
2. What's degree should I set on the sub so that it goes well with my main speakers?
3. How's important of phase? Does it matter if I set 0 or 180 degree?
Thank you very much
Actually, DT, you have found the solution to all your questions just by experimenting. But you need the technical reasoning behind it all, which I will try to help with.

1. Phase is whether the speaker cone is moving out with a signal that is also moving out. Think of a trumpet blast as an outgoing phase launch, one can feel that air move towards you. Another example is talking vs. talking while breathing in (yes, it sounds weird). You might say it should always be 'in' phase or 0, but some electronics change the absolute phase of the recording simply by the number of stages of amplification they have in them (even - in phase, odd - out of phase).

2. This is the easy one. You want the one that "fights" less with your main speakers, meaning your speakers are in one phase and your sub in the opposite, creating cancellations. And that means....more bass output. So 0 is correct.

3. It depends on your ears; if the crossover is fairly high, you may hear a hollowness in the music. But set it at 0 and you'll get to listen to all the bass you paid for.

Hope this helps!

Tsouthworth is right on the money. My preamp inverts the signal so I must swap speaker leads minus to plus and plus to minus. With your setup I would only have to flip the switch.
worldcup...for a detailed explanation to your questions let me refer you to lars fredell's article on the subject in the summer 2000 issue of ultimate audio mag. the terms 'phase' and 'polarity' are used interchangably most of the time.when cds or sacds are recorded they are recorded either in 'phase' (absolute polarity)( 0 degrees) or out of 'phase' (reversed polarity)(180 degrees).recordings that are 'in phase' or absolute polarity' will have deep taut bass good sense of air and transparency and dimensionality.those out of phase ( reversed polarity) will have bass that is not as deep or taut,sibilance might be too pronounced and strings too glaring.( i'm quoting lars here)the 0 degree and 180 degree designation on your sub allows you to dial in absolute polarity or switch it 180 degrees. some record companies cds and sacds (sony for one) are recorded out of phase.(the U.A. article has a list of recording labels and the usual polarity they record in).i would get the mag. and refer to the list when playing cds or sacds and play with the switch. your ears can usually,like yours already have,tell when a recording is 'in phase' or 'out of phase' ( absolute polarity or inverted polarity) hope this helps a little. dave smith 'calloway'
Yo Worldcup86, Phase in simple terms is the direction a driver cone is moving relative to the electrical signal being sent to it. In a perfect world, all the drivers in a speaker system would move out at the same time and in at the same time. That way they would be "in phase" with each other. But in the real world, we have crossovers (inside the speakers) that change the phase angle of the various drivers relative to each other. In extreme cases, if the phase angle between two drivers is 180 degrees apart, then they will cancel each other out at overlapping frequencies. This will cause a dip in the frequency response in the region where the crossover point is. Depending on how low your main speakers will go and where you set the rollover frequency on the sub, this region can be fairly large or very small. Lets suppose that your main speakers put out bass down to 50hz. and you want to achieve the flattest frequency response possible. If you set the sub crossover point to 40-45hz then the sub will be rolling off about 3db (half as loud) at 50hz. The main speakers will be rolling of about the same amount and you'll have enough overlap that you shouldn't have a big bump or a big dip at 40-50hz but this is also quite dependent on the room characteristics and sub placement. Once you have the crossover point set, then you can experiment with phase setting. Ideally, you want the cone of the sub moving out (toward the room) when a bass note, gunshot, explosion, or any other loud sound occurs. That will provide the most "felt" impact. If the cone is moving in (away from the room) then the air in the room is not compressed at the right time and some of the effect is lost. Once you get the sub in phase, then the main speakers need to be checked as well. You can change the phase by reversing the speaker wires (red on black) if you need to.
when the audio signal goes through an electronic crossover in either in the sub or processor, the phase of the signal is changed. The phase contol on the sub allows you to readjust the phase of the sub so that it is in phase with your main speakers. When the two are in phase, the bass will be louder. Ideally, the phase contol should be adjustable from 0 to 180 degrees not just 0 and 180.