My understanding of it is this:
* The correlation meter measures the linear correlation between left and right signals. So if the signal at one point is 100% on the left and 100% on the right, the correlation is 1. If 100% left and -100% right it's -1 correlation. Etc.
* So the correlation meter says something about the stereo separation. If correlation is always 1 then there's no stereo separation: It's mono. If the correlation is always -1 then there's also no stereo information, but the channels are in inverse polarity (meaning the track will be silent when playing back in mono).
* Remember most of the energy of audio is in the low-end of the spectrum, so the correlation meter correlates mostly to bass.
* So you could say the more 'stereo' it is, the more the correlation meter will go towards 0.
* Also remember that bass frequencies have very little directional information in it (because the waves are so long). There's a reason why it's rare to see more than one sub-woofer, and if there are they likely get the same signal.
* But depending on the music, you'll probably want to keep it on average well above 0 because chances are that bass frequencies might often be cancelling out between left and right otherwise. And again, most of the stereo image is not in the bass; it's in the higher frequency ranges.
Personally I start worrying if the correlation meter is all over the place and often gets even well below 0. Because that usually means to me that I have an issue in the bass. Maybe a panned bass-heavy track is out of phase or something. Maybe it means I have to look at stereo effects (such as reverb) on bass-heavy instruments (such as bass guitar, kick, synths, or even guitars).