set transaction isolation level serializable is the highest form of locking. It requires reads of data to be repeatable. Any data that is being updated will not be included in your queury. set transaction isolation level read committed is one level below the above. It allows queries to use data as long as it has been commited. This would include data in the redo log.
Highest form of locking? The command has to do nothing with locking. It only sets multiple selects from a table to bring up the same information before the transaction is commited. "Set transaction isolation level serializable" influences only the next transaction. After you commit the transaction, the isolation level reverts to the default "read committed".