Some of the payment gateways integrated with HostBill require to setup a Callback URL. (It can also be named Webhook / Notify / IPN URL depending on the gateway).
The Callback URL address is used for payment gateway to notify HostBill module about payment status change (for example to notify that the payment has been captured). HostBill then uses the information to generate the transaction, update invoice status etc.
Callback URL in HostBill is generated automatically using ID of the payment gateway module in HostBill. Not all payment gateways require Callback URL but it is displayed for each payment module activated.
To find out where to insert Callback URL in gateway panel please refer to the specific payment gateway documentation article.