It means your shipboard account will have a credit issued to it when you board. So if you have a $100 onboard credit and spend $500 while on the cruise, you'll only have to pay for $400 of it, either by cash or by a charge to your credit card.
...like a gift card that can be used for anything on the ship (and expires at the end of the cruise)....stuff from the shops, drinks from the bar, massages, dinner clubs, excursions, etc.