Advanced balance(to customers) is the sum of the amount that the business should pay to 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 add the required amount value and click on the save button.
To make a purchase or sale using this advanced balance, add a sale or purchase. Select the customer/supplier to whom you have added the advanced balance. While making the payment select the advance option and save it.