Status can be one of the following:
A READY status means that the instance can accept connections.
A BLOCKED status means that the instance cannot accept connections.
A READY/SECONDARY status means that this is a secondary instance in an Oracle Real Application Clusters primary/secondary configuration and is ready to accept connections.
An UNKNOWN status means that the instance is registered statically in the listener.ora file rather than dynamically with service registration. Therefore, the status is not known.