It means that a patient's symptom was treated successfully, but then later returns. So if you have a headache and you take some ibuprofen to get rid of it, then the headache comes back, it is said to be a rebound headache. Another possibility is pain that is felt upon removal of palpation pressure. So if your abdomen is being palpated, and if, when the pressure of palpation is removed, it is painful; you are said to have positive rebound.