When you REDO a transaction it means that the changes made by a transaction that has been completed successfully are redone. The status of the database is reset to the consistent status it had at the end of the transaction.
The database system stores all the information required for the REDO in the redo log entries (also known as after image entries.) Redo log entries are stored permanently in the log area only after the COMMIT for the transaction.
See also:
Using Log Entries at Restart or Recovery