An IP address is a unique number which separates a website or a server on the Internet, so if you have a dedicated IP, it'll be used only by your sites and won't be shared with others as it happens with shared hosting accounts. Provided you have your own hosting server, you will have a dedicated IP, but you may need additional ones for a variety of uses. Provided you have a web-based store, for instance, you'll need an SSL certificate for it, so as to make sure that the payment information your clients submit will be encrypted and safe. The same is valid if you have a login form of some sort and you would like the usernames and the passwords which visitors enter to be protected. The SSL certificate requires a dedicated IP, which has to be different from the one which you already have on the machine. You could also need a separate IP for an app such as a VoIP server, or if you desire a slightly better functionality for a given Internet site, which will affect its position in search engine results.