Scientifically, 1 kilocalorie (1000 calories or 1 kcal) means the energy it takes to raise the temperature of 1kg of water by 1°C.
Calories are units of energy so small that a tiny cookie can provide thousands of them. To ease calculations, energy is expressed in 1000-calorie units known as kilocalories. That is, 1 Calorie is equivalent to 1 kilocalorie; the capital C in Calories denotes kcal on food labels, calories and kilocalories are used interchangeably to mean the same thing. In other words, it is just as normal and acceptable for people to use the small “c” instead of big “C” and say “1 gram of fat gives us 9 calories” in the nutrition world as “1 gram of fat gives us 9 kilocalories or 9000 calories” in the physical science world.