Clarification on How This Correlated Subquery Runs (Guided Project: Customers and Products Analysis Using SQL)

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#1

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My Code: 


 WHERE p1.customerNumber NOT IN (SELECT customerNumber
                                   FROM payment_with_year_month_table p2
                                  WHERE p2.year_month < p1.year_month)

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I can't seem to wrap my head around what this piece of code does as the output I'm getting is opposite to the output I'm expecting. Could anyone help with explaining what this code does? Thanks