To go short, check the pictures, it’s pretty straightforward …
A signal can be made of 2 distinct channels : left & right.
When the signal from Left is heard on the Right channel , the sound gets blurry.
The signal is out of phase, like a phaser effect, but not intended.
When a signal is 100% aligned, if you invert 1 channel, the sound cancel & no sound is heard, that’s a signal In Phase, so the correlation is @ +1.
Now, insert a very short delay time on either left or right channel & you’ll hear what is a signal out of phase.
The correlation meter then approach the 0 value.
The sound is very recognisable , it’s blurry, not accurate at all.
A (mono) signal , in phase.
A stereo signal in Phase.
A signal out of phase.
In some cases, you can play with the correlation, the stereo width, the phase … you can get some interesting results … but 1st, understand the subject, try it yourself & train your ears so when you know what Phase is, you can break the rule & have fun with it …
Hope, my non professional explanation is not out of phase