IMO you shouldn't use the word "phase" when talking about speakers, you should use "polarity". But that's another discussion.
Loudspeakers work by moving their cones in and out which in turn creates pressure waves in the air. The cone is moved by running an electric current through a coil which causes the magnet inside the coil to move in and out, and the magnet is attached to the cone.
(Yes, I know this is simplistic).
The "problem" is that if you run the current through the speaker one way the cone moves in, but if you run the same current through the other way then the cone moves out.
This doesn't matter if you've only got a single speaker, but if you've got two or more speakers then they all have to be connected up the same way round. Imagine you have a pair of stereo speakers connected up, and one is connected the opposite way round from the other. Now imagine you feed the same signal into both speakers. The movements of one speaker will be the exact opposite of the other one - going in when the other goes out and vice versa. When the two resulting soundwaves have travelled to where you are listening, the high pressure part of the wave from one speaker will coincide with the low pressure part of the wave from the other speaker, they will cancel out, and you won't hear anything.
In pratice you rarely get exactly the same sound fed to two different speakers and the listening position is usually slightly off, so you'll never hear perfect silence, but even so when the speakers aren't all connected the same way round it sounds very peculiar. Assuming all the speakers but one are the same way round then you would describe the remaining one as "out of phase". Or if two speakers are connected differently you'd say that "they are out of phase".