When you slide the "Marked as Picked up" button (or Encode on POS for dine-in orders), this means that you have finished processing the order on your POS and would like for the order to disappear from your tablet.
Furthermore, when you slide "Marked as Picked up" for delivery orders, this will notify the customer via SMS that their order is on the way.
If you are not done processing order but have accidentally selected this option, you can view the order again by going to Recent Orders and selecting the completed order there.