A repeat customer is a paying customer who has come back to your store to place additional orders after their first.
Getting one-time customers to come back is such a huge hurdle for ecommerce stores that it's important to track customers who have gotten past that.
Repeat customers are the same as returning customers in my opinion. Some people make a distinction between the two like returning customers have placed two to four orders and repeat customers have five or more orders. I feel like that distinction doesn't matter enough to give different labels to each.
Eric Davis
Get a snapshot of your customer behavior
The sooner you signup for Repeat Customer Insights, the more comparison points your store will have. Each 1st of a the month a new snapshot is taken and saved to your account for analysis.