Your confusion ends here, @dcam.

Allow me to fill you in on everything about the amount showing in the Total column on the customer's page.

As mentioned by my colleague Ryan_M, the column shows the total open balance of your customer. For example, if you have open invoices that aren't paid yet, the total amount of those invoices will show on the bottom. Thus, when the invoice gets paid, it'll deduct from the open invoice. 

Also, you can run the Customer Balance Detail report shared by my peer above to see the total open balance for all your customers.

You might want to check out this article to learn how to manage your customers in QuickBooks Online: Add and manage customers.

Please let me know if you need clarification about this, or there's anything else I can do for you. I'll be standing by for your response. Have a great day.