A correlation meter will measure and report on the phase relationship in a track.
A '0' or middle readout on the meter indicates that you've got a wide mix.
A -1 readout indicates phase misalignment and that the track will have issues when played in mono, such as frequency masking.
+1 = Mono signal (information that matches in both the L + R channels, or, the information that's in correlation.)
-1 = Stereo Signal (information that does not match between L + R channel or the information that's out of correlation.)
0 = In between the two above values, not entirely in phase or out, somewhere in the middle.
The +1 measure is assessing the difference between the two L & R channels; when they match, we have what's called a positive correlation.
The 0 measurement indicates a wider stereo field, so a wider discrepancy between the L & R channels.
Then the -1 measurement tells us that the L & R don't match at all and that the track won't work in mono, at least, not to the extent it does in stereo. By won't work, I mean some sounds and frequencies will be missing, masked, or both. The -1 measurement is emphasising the mono-compatibility risks you're taking.
Back to the tutorial notes:
When two waves perfectly align, you will hear both sounds.
When a wave is 180 degrees (an exact invert) out phase with another, they will cancel each other out, and you will hear nothing. Add a sample into your D.A.W., duplicate it, and flip the phase of one, 180 degrees. Upon playback, you shouldn't hear anything.
When two different waves play together, a new wave is born from a mix between the two.
You can nudge a waveform (instrument/sound) in your arrangement page to try and align the waves.
MV: Some tools perform this automatically, such as MAutoAligin by Melda).
Out of phase sounds like a chorus or a washed-out effect.
Phase issues can occur without showing up on a correlation meter. If the meter is on your master output and measuring the whole project, small issues on single tracks will not be apparent.
EQ's impart phase smearing into sounds, so use them sparingly or use the linear phase options.
Culprits for causing phase issues:
Anytime you have a duplicate sound in a mix, two of the same, or a slightly differing versions playing together in a project.