Install an additional oracle database on one node of an oracle cluster (RAC)

From the documentation:

Oracle RAC One Node is a new option available with Oracle Database 11g Release 2. Oracle RAC One Node is a single instance of an Oracle RAC enabled database running on one node in a cluster. This option adds to the flexibility that Oracle offers for database consolidation while reducing management overhead by providing a standard deployment for Oracle Databases in the enterprise.

This is a means to have a multi-node RAC, one which one database can only run on one node at a time, so it allows you to (more cheaply) use RAC for active-passive clustering, which saves on having both RAC and another clustering product such as VCS. Or are you asking if it is possible to have a RAC system entirely on one physical host, running Clusterware, ASM, etc and a database? Yep, that is possible too, the standard installer can do this for you.

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.