Several scripts require a hostname (the server name) when they are installed. You can use localhost as hostname. This will tell the script that the MySQL server is on the same machine as the script, which is the case most of the time.
If you have already purchased a domain name, you can also use the fully qualified domain name (without http: // or www: //) instead of localhost. In 99% of cases, the fact that you choose localhost rather than your domain name makes no difference.