Rebuy Smart Links are used to create pre-built carts with desired items that can be sent out in things such as email campaigns. These are in the format of a URL and can be configured with specific arguments to achieve desired outcomes.
An example would be a URL that is sent in an email campaign. When the customer clicks on the link, they are taken to your store where with items X, Y, and Z automatically added. Then they land on the Shopify checkout page with a discount code automatically added so that they get a nice little discount.
Or, if you are using the Smart Cart, you could have the items automatically added then have the customer land on a specific page like a collection page. The Smart Cart opens to show them the items that are added for them. The discount code added to the Smart Cart to show them their discount. And, a banner with a message to let them know that they just unlocked a 10% discount where they can add any other desired items from the specified collection.
Merchants have even used these links embedded into the HTML/liquid of their home page. They offer a bundle of items and when clicking the Call to Action the customer gets all of the items added to their cart and they are redirected to the Shopify checkout page so that they can easily complete the order from there.
For more in depth information on the admin setting from the Rebuy Admin Dashboard please visit this article here.