What is out of phase for a speaker?

In general, speakers have two connections, and it is important to connect them the "same way" to the amplifier. (Amps also have pairs of output terminals.) We usually designate the terminals of the speaker as positive and negative. (Outputs of amps are usually designated the same way.) It is important to connecteach positive speaker terminal to the positive output of the amp for a given channel, and each negative speaker terminal to the negative output of the amp for a given channel. This way the speakers will be connected in phase. It is possible to reverse all of the connections and not have problems, but it is critical that speakers be connected in phase.

Speakers do not become out of phase or change phase. When they are constructed, their operation is fixed and will not vary during their lifetime. A given polarity of a wave, either positive- or negative-going, will always cause the speaker to move in the same given direction.