A dedicated IP (Internet Protocol) is a unique Internet address dedicated exclusively to a single hosting account. Normally, several hosting accounts reside on a single server and share that server's IP address. Dedicated IP addresses are usually used for large SSL-encrypted secure websites and particularly large and/or traffic-heavy sites. Most shared hosting users have no particular need for a dedicated IP address.
However, a dedicated IP address for your hosting account enables you to:
Why Do I need a Dedicated IP for SSL?
When securing some connection with TLS, you usually use the certificate to authenticate the server (and sometimes the client). There's one server per IP/Port, so usually there's no problem for the server to choose what certificate to use.
HTTPS is the exception — several different domain names can refer to one IP and the client (usually a browser) connects to the same server for different domain names. The domain name is passed to the server in the request, which goes after TLS handshake.
Here's where the problem arises - the web server doesn't know which certificate to present. To address this a new extension has been added to TLS, named SNI (Server Name Indication). However, not all clients support it. So in general it's a good idea to have a dedicated server per IP/Port per domain. In other words, each domain, to which the client can connect using HTTPS, should have its own IP address (or different port, but that's not usual).
NOTE: Our high-end SSL certificates include a dedicated hosting IP, so you do not need to purchase a dedicated IP credit separately if you are adding a certificate to your site.