Group users by when they joined.
A cohort is everyone who started in the same week or month. Following each group over time shows whether the product is really getting better.
Falls to zero
Nobody sticks. Fix the product before growth.
Flattens
A core group keeps using it. A sign of fit.
Smiles
Users come back and grow. The best shape.
Source: Andreessen Horowitz ↗
Averages hide. Cohorts reveal.