A COMMIT concludes a transaction/subtransaction. When a COMMIT is performed, all changes made by the transaction are stored permanently. Any changes that are completed by a COMMIT can no longer be reversed by a ROLLBACK.
After a COMMIT, the database system releases all blocks that have been allocated to the transaction and opens a new transaction implicitly.
Usually, the database system performs the necessary COMMIT actions itself. If the AUTOCOMMIT mode is switched off, you can request a COMMIT explicitly by executing an appropriate SQL statement.
See also:
Reference Manual, COMMIT Statement