If you have an extension for Magento 2 in your cart, you will be prompted to enter a domain name of the store where the extension will be installed, e.g. "http://my-store.com"
If you've got a multistore environment, you need to enter a base URL, e.g. if your backend login page has an address "https://thebestshop.com/index.php/admin", you need to enter "https://thebestshop.com"
In case you are not sure what the final domain name will be but already have a development store, its URL can be specified as well. However, don't forget to contact support and ask them to change the URL attached to your license as soon as it will become known for you.