The metric of the cohorts analysis shows, what percentage of the new visitors of a defined month returned per month.
Use the metric with the dimension "Cohorts (Monthly)".
Month 0 = Returning visitors within the first 29 days. Is always 100 %.
Month 1 = Returning visitors within days 30-59 since the first visits.
...
Calculation: "Visitors (Lifespan Month N)" / "Visitors (Lifespan Month 0)" * 100
Extensive information on cohorts in Webtrekk can be found in the corresponding training chapter.