A retention curve plots time on the x-axis and the percentage of an original cohort still active on the y-axis. The shape of the curve diagnoses the business: a curve that decays to zero means no PMF; a curve that flattens at a stable plateau means real, sticky usage; a curve that turns upward (the 'smile') means net-negative churn and is the strongest signal a product can show.
Investors read retention curves before any other metric on the Traction slide. A strong curve compensates for weak top-line growth; weak curves invalidate strong top-line growth.