The metric of the cohorts analysis shows, how the average order value per month developed since the first visit.
Use the metric with the dimension "Cohorts (Monthly)".
Month 0 = The order value generated on average within the first 29 days.
Month 1 = The order value generated on average within the 30.-59. days since the first visit.
Calculation: "Order Value (Lifespan Month N)" / "Qty Orders (Lifespan Month N)"
Extensive information on cohorts in Webtrekk can be found in the corresponding training chapter.