If you eat food that was left out for too long the food could cause you to get food borne illnesses or even just plain food poisoning.
If the food requires refrigeration and the food is left out for longer than 2 hours without refrigeration then the food can grow bacteria in it that you may not be able to see.
When that food grows bacteria it can make you sick and may even potentially lead to botulism which can be deadly.
Always refrigerate food within a few hours of cooking it or if you have leftovers refrigerate them within a few hours of finishing your meal.
If the food that requires refrigeration such as noodles, casserole, meats, hot dogs, hamburgers etc have been left out longer than a couple hours then it might be safe to eat but it would be at your own risk.