Each domain name that's registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if needed. As an example, the A record of a domain address is its IP address i.e. this record reveals where the web site is located; the MX records reveal where the emails are stored, while with an SRV record you can use a domain name for unconventional purposes, for instance a Voice-over-IP service. To be able to create and modify particular records enables you to have different services with different companies if, for illustration, your present host provides good web hosting, but has troubles with the email service, or you simply prefer to get a certain service from a certain company. While you should be able to create and modify these records freely and with no limitations as you own a given domain, lots of companies charge extra to give you this option.