The manual explains what is does:http://docs.oracle.com/cd/E11882_01/ser ... SQLRF50502
Hints tell the optimizer to do something that it may not otherwise do. As a result, in some situations a hint may improve performance, in others is may reduce it. You should try not to use hints unless you have a documented case where it improves performance. Even then, you should attempt to remove them with subsequent patches and updates to see if the particular issue you faced has been resolved in the updated optimizer.