A Database Definition file contains attributes defining the database. The database definition file must use .dbdef as the file name suffix. The name of the database definition derives from the name of the database definition file. Here are the attributes used for this local machine demo database. You should change the directory of the DataStore to fit the environment of your location machine where you would like the database files to be located. Also, note the location of the .dbdef file is under directory /grid/config.
Once this file is created, you are now ready to go to the next step in the quickstart blog.
Disclaimer: These are my personal thoughts and do not represent Oracle's official viewpoint in any way, shape, or form.