In customer analysis, Whales are the customers who are placing large orders with your store. Their orders are big enough that you probably double-check that the order is legit.
You might think you can just sort your customer list by the total spent and that'd be enough, but you'd run into problems that way.
What about the customer who has ordered below your Average Order Value in every order but they've ordered two dozen times?
They are showing loyalty but with below-average order sizes, they aren't a whale. Their behavior is completely different than whales (high volume, low amounts VS high amounts). They should be handled differently, with different messaging and different incentives.
The best way to find your whale customers is to compare their total spent against their number of orders. This will also be the customer's Average Order Value (not to be confused with your store's Average Order Value).
Sort your customer list by that and you now have your whales.
Segmenting your whale customers and advice on how to reach them is included in Repeat Customer Insights as part of the Automatic Segments. It'll analyze all of your Shopify customers for you and make it easy to find your whales (and other segments).