A kcal is a kilocalorie which is a unit of energy. In non-scientific diet books it may be referred to as a Calorie (with a capital C). It is roughly equivalent to 4.2 kilojoules (another unit of energy).
They used to call them just 'Calories' - with a capital C, and the smaller unit, i.e. 1,000 times less, just a 'calorie'. Finally they realised this was stupid and decided to standardise it to bring it into line with just about every other metric measure, so changed it to be Kilocalories.