Each cell is the Pearson correlation of the two assets' daily log-returns over the lookback window. +1 = they move in perfect lockstep, -1 = they move oppositely, 0 = no linear relationship. Pros watch this to avoid stacking correlated risk and to find pairs trades. Each pair is matched on the session date — only days both assets printed are compared, so a shorter history never gets lined up against an older window of its counterpart.