Kind of hard to explain. Let's see ummm... phase determines the polarity (+ and -) of the speaker connections. 0 is normal... woofer fires out, 180 is reversed.... woofer fires in. On a variable phase control you can adjust anywhere between these two points. It is mainly used in home setups to get the best performance and output of a subwoofer and tame the effects of phase cancellation between a subwoofer and the main speakers. In a typical car install, you will want to leave it at 0 unless the subwoofer is inverted.