Essentially the RMI registry is a place for the server to register services it offers and a place for clients to query for those services. See Introduction to Java RMI. Excerpt:
Figure 1 shows the connections made by the client when using RMI. Firstly, the client must contact an RMI registry, and request the name of the service. Developer B won’t know the exact location of the RMI service, but he knows enough to contact Developer A’s registry. This will point him in the direction of the service he wants to call..