When you install a web application on your hosting account, you will often be asked to provide your MySQL server's hostname. Generally, you will be installing your web app on the same host on this the MySQL server is installed, so you can use: localhost.

If you have purchased a domain name, you can also use your complete domain name (without the http:// or the www://) instead of localhost. In 99% of the cases, whether you use localhost or your actual domain name will make no difference.