What's Mean By Redo Log?

The most crucial structure for recovery operations is the redo log, which consists of
two or more pre allocated files that store all changes made to the database as they
occur. Every instance of an Oracle Database has an associated redo log to protect the
database in case of an instance failure.

Redo records are buffered in a circular fashion in the redo log buffer of the SGA and are written to one of the redo log files by the Log Writer (LGWR) database background process. Whenever a transaction is committed, LGWR writes the transaction redo records from the redo log buffer of the SGA to a redo log file, and assigns a system change number (SCN) to identify the redo records for each committed transaction. Only when all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed.

Redo records can also be written to a redo log file before the corresponding transaction
is committed. If the redo log buffer fills, or another transaction commits, LGWR
flushes all of the redo log entries in the redo log buffer to a redo log file, even though
some redo records may not be committed. If necessary, the database can roll back these
changes.