Hello @MelLindaB 

Amount due shows you overdue balance for the customer. Balance field is showing all your receivables whether they are overdue or not.

See description from help.

Balance:
The customer’s current account balance in accounting currency. If the Payer Perspective check box is selected, the balance includes the open amount of all ledger items for which the selected customer is entered as payer.  
 

Amount Due:
The amount due for payment per current date is displayed. If the Payer Perspective check box is selected, the due amount of all ledger items for which the selected customer is entered as payer is included.

In balance per currency tab you can see balances divided per currency.