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
Analyze your customer behavior
A quick and automatic way to measure your customer behavior is with Repeat Customer Insights. It'll automatically import your Shopify data and run a number of analyses to highlight different customer behavior.