In most cases when a error is happening in Oracle you will get a ora-###### message and some text. However in some cases you will only receive the ORA error message number from Oracle and not the text. Most likely you would like to know the meaning of the error message. For this reason Oracle databases are shipping (for the Linux installations) with a tool named oerr which can be found in $ORACLE_HOME/bin when you for example want to know the meaning of the error ORA-06512 you can execute the command "oerr ora 06512" which will provide you the explaining text for this error message.
In case you run your Oracle database on a windows machine and you would like to have access to this tool, Yong Huang has created a perl script to run on your windows machine which can do the same and can be downloaded from http://orafaq.com/scripts/perl/oerr.txt .
Most likely you will not have to use the option very often however it can be very handy from time to time.