Domain name is the name that identifies your website. In reality the name you choose for your site points web surfers to the IP Address of the computer which is storing and serving up your web site.
The domain name comes in two parts. The actual name (i.e. selikta) and the identifier at the end (i.e. .com) When you register a domain name you are reserving that name for your use. Once you register it then nobody else will be able to use that same name unless you fail to maintain the registration (usually a yearly renewal).