As admin or reseller, you can manage resources. In other words, the IP addresses, MySQL servers, domains and of course your customers.
After the installation, froxlor should have automatically created IP address entries for you. Depending on your selection in the installation process, one for standard HTTP port 80 and one ssl enabled for standard HTTPS port 443.
If your server has additional IP addresses, e.g. IPv6, it is wise to add them all in froxlor. You can easily set one or more as system default in the settings, these will be pre-selected when adding a new domain for example.
For more details see IPs and ports.
You can now start to add your first customers. Fill out account and contact data accordingly, set desired service-resources and click Save. A customer is required in order to be able to add domains, as they are always connected to a customer.