Also called: physical correlation, correlation statistic
In plain English
Correlation measures how closely the physical order of rows on disk matches the sorted order of a column’s values, on a scale from -1 to 1. A value near 1 means the table is almost already in that column’s order.
Why it matters
It tells the planner whether an index range scan will read disk pages in near-sequential order (fast) or jump around randomly (slow). High correlation makes index scans much cheaper — which is exactly the order that CLUSTER tries to create.