NRR measures how much revenue a company retains and grows from its existing customer base over a 12-month period, including the effects of expansion (upsells, cross-sells, seat growth) and churn. An NRR above 100% means the existing customer base is growing without adding a single new logo — a compounding growth engine. NRR is arguably the single most important metric for SaaS business health because it determines how much new ARR is needed just to stay flat.