What does this Android code do?

DictionaryDatabase is your class that extends SQLiteOpenHelper (your database)
now you want to open your database and (read) or (insert, update, delete) from it to this, you have two options

 SQLiteDatabase db = dbHelper.getWritableDatabase();//used for insert, update, delete operations

or

SQLiteDatabase db = dbHelper.getReadableDatabase();//used for read (select) operations