How do the re loadable debit cards you can buy at retail store work?

You hand over cash & your card to the store - they credit the card with the amount of cash. You can 'spend' that cash at any time up to the amount on the card. If you want to buy something more expensive than the available funds - you can pay the balance owing in actual cash. Once the funds are used up - you simply repeat the process to add more money to the card.

A more 'user-friendly' debit card - is one that's linked to a bank account. That way you don't need to keep topping it up - because the funds for any purchases are simply debited from the linked bank account.