about defined('BASEPATH') ?

Normally every php file can be run which can reveal parts of your site or cause other unwanted output like error messages if that file isn't meant to be run from the web.

The line with BASEPATH prevents naughty people to get output from files that are only to be used by the framework like php files in your model and views folders. 

Beware, this doesn't mean it makes your site unhackable!! It just maybe prevents some people from getting easy useful information which can be used for hacking your site.