Attracted: Number of contributors entering the project for each cohort. In charts, cohorts are organized by "age" (time since they entered the project), each bar representing one of these cohorts. Large bars for any age mean that the community was attracting many contributors that time ago.
Retained: Number of remaining contributors (those still active) for each cohort. Large top bars (corresponding to older cohorts) mean that experienced contributors are still active. Large bottom bars mean that new contributors are being retained.The ratio between retained and attracted contributors for a cohort represents the retention ratio for that cohort (fraction of contributors of that age that are still active).