Vectorscope ​

A vectorscope is a useful visualization tool that plots the amplitude of a stereo signal in a two-dimensional X-Y graph, with X being channel one horizontally and Y being channel two vertically, thus revealing the relationship between the two signals (correlation).

Simplification ​

Simply put, the vectorscope shows how "wide" or "stereo" the signal is.

Correlation ​

Correlation is an arbitrary value that defines how 'similar' the two signals are, meaning a stereo signal of an identical sinewave with synchronized phase would result in a correlation value of +1 whereas a signal with one of the phases inverted (180°) would result in a correlation of -1.

Visually we can understand what the vectorscope is showing us by understanding that when the lines are mostly vertical we have a correlation near +1 versus when the lines are horizontal which means we are near -1.