Don't hafta shout, dood..
"Phase" is the relationship between AC signals - in the case of audio, it's the relationship between sound waves.
"In Phase" means 2 signals (like left and right channels) are both moving in the same direction, either + or -, at the same time.
Phase is measured in degrees. 0 deg. is the same as "In Phase" - there's 0 degrees of difference. 180 degrees is when the 2 signals are exactly opposite each other.
The reference is usually to speaker polarities. You want all speakers to be "pushing out" or "pulling in" at the same time. If one is going out and the other is going in (180 degrees out of phase) they will cancel each other.
That's why it's important to put the + and - wires from your amps to the corresponding terminals on the speakers. You want all speakers to be in phase.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's