One of the main reasons you might use a customer cohort report is to understand how a group of customers behave over-time. Most of the time it'll even be to understand how a group of customers acquired in a specific month behaved.
The way the cohort reports are arranged, you'd just need to read the row from left to right. Typically the initial month is first, followed by the second month, third month, etc.
Repeat Customer Insights adds some extra data to its cohort reports to make the analysis easier like totals or averages.
When reading a cohort report's row you're looking for odd data or trends. It's natural for most metrics to fall off over time (e.g. customers slowly stop coming back). Big spikes up or down are signs of major events to look into. It could be any number of things: a sale, they ran out of product at the same time, a seasonal time, etc.
The cohort report is your first step in the customer analysis. You want it to give you clues and ideas to start tracking down why something happened.
Just remember, reading the row is showing the behavior of the same group of customers over a stretch of time.
Eric Davis
Customer behavior analysis for better Shopify store performance
The Shopify App that increases repeat customer purchases through customer behavior analysis.