Each domain name which is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be created manually if required. For instance, the A record of a domain is its IP address i.e. this record reveals where the web site is situated; the MX records reveal where the emails are managed, while using an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. To be able to create and modify certain records permits you to have different services with different providers if, for instance, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a certain service from a specific company. While you will be able to create and modify these records freely and with no limitations as you own a given domain name, a lot of companies charge extra to give you this option.