I've bought one of these from another seller.

It's an e-gift card code. You can use a phone app like Android Pay or Gyft to store the code. On checkout day, the hotel presents you with a bill. When you are at the front desk checking out, you can tell them you'd like to pay the bill with a gift card.

Thankfully there is a way to check the balance of the gift card so I personally called and checked balance immediately prior to checkout to verify that it was still a working card.

There is no way to pay online or anything. You will have to reserve the room normally with a credit card, and present a credit card at check in. The hotel had already done a temporary authorization on my credit card for the full amount but they never actually charged the card anything and the temporary authorization dropped in a couple of days.

I was also able to charge food to the room during my stay. It made it a pretty nice affordable trip