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.
On a MultiSite-enabled plan, each subaccount can have their own dedicated IP address whereas on non-MultiSite plans, all domains share the dedicated IP.
A dedicated IP address costs $2/month in most locations (Australia and Singapore being the exceptions), and some plans come with a dedicated IP address bundled into the price. All dedicated IP address requests require ARIN justification, so if you have any questions, please contact firstname.lastname@example.org for clarification!