A COMMIT concludes a transaction. When a COMMIT is executed, the database system saves all the changes carried out by the transaction. The system can no longer use a ROLLBACK to reverse any changes that are completed by a COMMIT.
After a COMMIT, the database system releases all locks that have been allocated to the transaction and implicitly opens a new transaction.
System default: the database system automatically executes all the necessary COMMIT actions (AUTOCOMMIT mode is activated). If you deactivate the AUTOCOMMIT mode you have to explicitly specify a COMMIT statement.
SQL Reference Manual, COMMIT Statement