A dedicated IP is a unique address dedicated exclusively to your single Site5 hosting account. This is a feature that is required if you wish to set up a dedicated SSL certificate. Also it comes in handy when you need to view your web site, without the use of your domain name. Without it, you would be dependent on using the shared IP address of the server you are on.
A dedicated IP address costs $2/month or $24/year. Some plans come with a dedicated IP address, so if you have any questions, please contact sales@site5.com for clarification!