The waiver is a £10 add-on which drastically reduces the potential costs for equipment loss/damage/theft, that you may have in resort.
The waiver allows you to collect your hire without a bank credit card swipe, and if you have any issues with your equipment, you can replace the affected items without having to front the big costs there and then.
Your waiver covers you for one full set of equipment. So, you can lose your skis one day, your boots the next, and the same waiver can be used for both these items. The waiver will not cover you for the new equipment you receive; you will need to purchase a new waiver from the shop.