In said file, you have the following :
define('PAYMENT_CRYPT_KEY', 'randompasswordchangethis');
You need to change 'randompasswordchangethis' to whatever you like, for example
define('PAYMENT_CRYPT_KEY', 'lisuayneriauerbwerklcxr');
code]
Your Paypal's API details are stored in the database, if someone steal your database it could access your paypal account and withdraw the money. changing said string, your API details will be encrypted, so in the case they steal your database, they will need to know the key you used.