It adds a Reorder button to a past Real Time order's section on the order confirmation/order-details page (/checkouts/<id>), next to that order's title. Clicking it opens a "Reorder" popup: pick which of that order's meals to include (checkboxes), choose a new date, then either Edit options (adjust portion sizes before adding) or Go to Cart to add the same items straight to your cart for the new date. With the setting set to No (the default is Yes), the Reorder button and popup are not shown on past Real Time orders. This setting lives on the Order: Realtime tab of Client Config, not on the ordering page itself.