If this constant is defined, Easy Digital Downloads will use PHP $_SESSION for storing cart data. By default, when it is not defined, EDD will use WP_Session, which is a combination of browser cookies and database rows stored in the wp_options table.
Place this constant in your wp-config.php file:
define( 'EDD_USE_PHP_SESSIONS', true );
If you wish to disable PHP Sessions, set it to false instead:
define( 'EDD_USE_PHP_SESSIONS', false );
If you are unsure of how to edit your wp-config.php file, please consult the official WordPress help page on editing the file.
Note: you must place this code above the line that says:
/* That's all, stop editing! Happy blogging. */