A web service registry is like a phone book. Given certain information, you can look up web service(s) matching the information.
This can be useful if an organization wants some flexibility regarding where its web services are located. Clients then use the registry to look up the (current) address of a service instead of having the address of the service hardcoded.
If the web service is later moved, then only the address information in the registry needs to be changed, in order for clients to find the web service.
Publishing a web service to a registry means creating an entry in the registry (again compare with a phone book) by associating some information with the address of the web service.