In PL/SQL the BEFORE UPDATE Trigger will fire before the UPDATE operation is executed.
CREATE TRIGGER trigger_name: To create trigger with sutable name.
REPLACE OR REPLACE TRIGGER trigger_name: To replace trigger with sutable name.
BEFORE: To specifies when you want the trigger to be executed. The INSTEAD OF clause is used for creating trigger on a view.
UPDATE: To specifies DML operation.
[ON table_name]: To specifies the name of the table associated with the trigger.
[FOR EACH ROW]: To specifies a row level trigger, Otherwise the trigger will execute just once when the SQL statement is executed.
WHEN (condition): The condition on which the trigger would fire. This is valid for row level triggers.