In Oracle® Database 2 Day + Real Application Clusters Guide 11g Release 2 (11.2), a server pool is defined as:
A server pool is a logical division of nodes in a cluster into a group to support policy-managed databases.
In Oracle® Database 2 Day + Real Application Clusters Guide 11g Release 2 (11.2), a Free pool is defined as:
A default server pool used in policy-based cluster and capacity management of Oracle Clusterware resources. The free pool contains servers that are not assigned to any server pool.
In Oracle® Database 2 Day + Real Application Clusters Guide 11g Release 2 (11.2), the section About Oracle Grid Infrastructure and Oracle RAC, says that:
The nodes in a cluster can be organized into a server pool for better resource management. Each server pool has the following properties:
Upon installation of Oracle grid infrastructure, a default server pool, called the Free pool, is created automatically. All servers in a new installation are assigned to the Free server pool, initially. If you create a new server pool, the servers move from the Free pool to the new server pool automatically.