Above 100% means the existing base grows by itself: expansion more than offsets churn, so the company would grow even if it stopped selling to anyone new. That property is what justifies high ev-sales multiples for software.
It is also the metric most vulnerable to definitional creativity. Check the cohort definition, whether usage overages count, and whether customers acquired mid-year are included.
Example: Northwind Cloud's customers paid $164M last year and $186M this year after $31M of expansion and $9M of churn, giving 113% net revenue retention.
Related: churn-rate, annual-recurring-revenue, rule-of-40, unit-economics, customer-lifetime-value