When the IP address of a domain, or a service for that domain such as email of ftp changes, it takes time for these changes to be updated to all the DNS servers throughout the world.
DNS Propagation is a process by which all of the DNS servers around the world collect this updated information directly or update themselves on how to find this informaton. DNS changes are starting to propogate more quickly than in the past, but it can still take up to 72 hours for the process to complete on some DNS servers that don't update so often.
When you change the settings for a domain using the "DNS Zone Editor" in the control panel, the change to the DNS settings is immediate and any new requests for that information will be available immediately. However, we cannot control how quickly these changes are reflected on other servers throughout the world as these other servers are not controlled by us.