The correlation meter indicates the degree of similarity (or correlation) between the left and right channels.
Visual Display
When the audio in the left and right channels is similar, the meter draws towards the top. The extreme case is when the left and right channels are exactly the same, in which case the correlation is +1 and the meter would be positioned all the way to the top.
When the left and right channels are different, the meter draws towards the bottom. The extreme case here would be for the left and right to be exactly out of phase, in which case the correlation is -1 and the meter would be positioned all the way to the bottom.
History
As the correlation meter updates, it "paints" a history to show the correlation of the left and right channels over time. Brighter regions indicate that the correlation meter has spent more time in that area. This provides you with a quick way to visualize the extremes of the phase correlation as well as the most common regions.
In general, most recordings have phase correlations in the 0 to +1 region. A brief readout towards the bottom half of the meter is not necessarily a problem but could represent a possible mono compatibility issue.
Note: As you apply greater multiband stereo widening or reverb width to your audio, the phase correlation will tend to draw more towards the bottom half of the meter, as the left and right channels will become "wider" or less similar.
Channel Operations
You can perform a quick check of mono and phase compatibility by clicking on the Channel button below the Vectorscope. This provides a menu that allows you to sum the output of Ozone to mono, invert the polarity of left or right channels, and swap left and right channels.
An additional meter for analyzing stereo spread is the Vectorscope.