Here is a more complete answer.
A PADI scuba certification course, as with all agencies, has three parts: academic learning, pool training, and open water dives.
In locations that might not have really suitable diving nearby for at least part of the year, it is common for students to do the first two parts locally and then travel to another location to do the open water portion. When they do that, their local instructor gives them a referral indicating completion of those two parts. The instructor in the remote location than provides a "referral course," which consists of a quick review to make sure you really did learn something, followed by the OW dives.