A DBA can be authenticated by two ways using the password file or through the operating system.
Let’s see what is a Password file?
If the DBA wants to startup an Oracle instance there must be a way for Oracle to authenticate this DBA. That is if (s)he is allowed to do so. Obviously, his password cannot be stored in the database, because Oracle cannot access the database before the instance is started up. Therefore, the authentication of the DBA must happen outside of the database.
How to create it?
C:\>orapwd file=D:\oracle\product\11.1.0\db_1\dbs\orarefer password=reference