All devices on the Internet are identified by a unique number called an IP address, such as 123.123.123.123. When you have a website, the domain that you enter to open it is to save you time, but the server where the site files are still has an IP. Because there're many more sites and devices than there're IP addresses, all shared hosting servers have a number of websites under the very same IP, while when you use a dedicated server you will have a dedicated IP too. Even in the first case though, you can obtain a dedicated IP for your websites and host them on a shared server. An advantage would be that you'll have superior search engine positions since a dedicated IP often means a quicker loading website. Last but not least, you need such an IP if you intend to acquire an SSL certificate for your site and protect the info that visitors submit on it.