Leftovers, as the name implies, most commonly refers to food that is 'left over' from a previous meal. This food is generally placed in a container like Tupperware or Gladware or wrapped in a storage medium like aluminum foil or Saran Warp and placed in the refrigerator.
While placed in a storage medium, leftovers exist in a quasi-stasis. While it is in this quasi-stasis, leftovers are like the Schroedinger's Cat paradox. Like Schroedinger's Cat, there are only two outcomes for leftovers, they will be eaten or thrown away. You don't known if you're going to eat the leftovers or throw them away until you remove them from the storage medium and check.
Leftovers can be eaten at any time of the day and may prove to be a good cube lunch or gut bomb.