Node and Instance Relationship on Oracle RAC
In general, it would mean that the instance that was servicing the request failed and is no longer running.  In a RAC cluster, you run one instance on each node of the cluster.  I wouldn't make sense to move an instance from one node to another in the cluster-- that would imply that one node would be running two instances of the same database.  Depending on your environment, the DBA might choose to add a new node (and a new instance) to the RAC cluster at least temporarily in response to the failure of one of the instances.