sounds "in phase" reinforce. sound "in quadrature" do not affect each other*. sound "out of phase" cancel each other.
phase typically refers to polarity of wiring or to the concept of a delay.
there is an excellent discription at betteraudio, but keep in mind that the terms "absolute" and "relative" phase both refer to different cases of "relative phase".
*quadrature sounds do not get louder, but there is an additional phase shift.