See where your mix sits between left and right. Mono stands on the middle line, wide sound fans out, and whatever lies along the edges is out of phase and drops away in mono.
The bar beside it shows the correlation: +1 is mono, 0 is fully wide, below 0 cancels in mono. With Band Correlation you see it for lows, mids and highs on their own, because width in the highs is fine and width in the bass is not. Click a band to see it on its own.
The edge turns red where a sample hits 0 dBFS.