DigitalConcepts is right.
All the information of the plugin is stored in the database, we encrypt that data with an additional hardcoded password/key. Said kay is in the oc-content/plugins/payment/index.php file
define('PAYMENT_CRYPT_KEY', 'randompasswordchangethis');
You need to change randompasswordchangethis to whatever you want to, so in the case a hacker has access to your database's content, it will not know what your paypal/other payments information is.
Why we do this?
Because we consider your payment information very important, if a hacker get access to your Paypal information for example, he could make payments or empty your account. So if you change the default password, the hacker will need access to your database AND your file server, which make things a little more complicated.