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).

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Real-Time Analysis

The vectorscope operates in real time, using the Fast Fourier Transform (FFT) to break down the signal into its constituent frequencies and plot their levels instantly.
The FFT size can be changed in Settings > Metering > Vectorscope > FFT Size

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Simplification

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

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Correlation

Correlation is an arbitrary value that defines how ‘similar’ the two signals are, meaning a stereo signal of an identical set of sinewaves with synchronized phases 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.