Calories are units of energy for your body. That's all. Not something big, bad and scary that should be avoided at all costs like we're told by the diet industry. They're kind of like oxygen – essential for us to stay alive. We don't judge oxygen as being good or bad, it's just necessary. Too much or too little oxygen and our body is in trouble, but our body also has mechanisms in place to defend against this, enhancing our survival.
In a literal sense, a calorie (technically I am referring to kilocalories) is the amount of energy it takes to heat 1 kilogram of water 1 degree Celsius. They are what our body derives from food that keeps our engine running. Without them we starve to death.
Again, calories in food are simply units of energy for our body. Why do we give them so much power?