A DigitalOcean Reserved IP address is a publicly-accessible static IP address that you can assign to a Droplet and then reassign to another Droplet later, as needed. You can implement a failover mechanism with reserved IPs to build a high availability infrastructure.
Reserved IPs are free when assigned to a Droplet.
Reserved IPs cost $5.00 per month ($0.00744 per hour) when reserved but not assigned to a Droplet, due to the shortage of available IPv4 addresses. You are not billed unless you accrue $1 or more per reserved IP.