Advanced balance(to customers) is the sum of the amount that the business should pay for the customers.
Advanced balance(to suppliers) is the sum of the amount that the suppliers should pay for the business.
To add an advanced balance, Go to Customers -> customers/suppliers and click on the action button. Here you will find a pay option and click on it. Add payment pop up will be shown and add the required amount value and click on save button.
Making sales/ purchase using advanced balance:
To make a purchase or sale using this advanced balance, add a sale or purchase. Select the customer/supplier that you have added advanced balance. While making the payment select the advance option and save it.