By default every country is available. By available we mean that a customer creating a new account can select it when filling in their address as shown in the following screenshot:
This is probably not what you want. There are likely many countries where you do not want shoppers to register from. For example, when based in Europe/USA, registering customers from Iran is unlikely to yield any benefit at the time of writing and many other more sensible reasons like the high cost of shipping to certain countries.
Removing countries is nice and easy.
Plan where you do want customers to register from and make a list in ...