what it does:
if defined, changes the icon next to the WPPizza text in the backend from the default to the given icon

affects:
admin/backend only

does not affect:
frontend or anything else barring the icon mentioned above

usable:
anytime, can also be changed or deleted again at any time.

example:

define('WPPIZZA_MENU_ICON', 'http://www.example.com/image.png');/*url*/

where to add/define:

in your wp-config.php before:
/* That's all, stop editing! Happy blogging. */

Disclaimer:
As with all filters, actions and constants:
Although these should be safe to use as indicated, there are no guarantees. Use at your own risk. Make backups before usage.