My son has a science GCSE exam on wednesday but one thing he doesnt seem to understand! Help me please.?

Well I just did my physics A level this week (fingers crossed =]) and the simplest way I can explain it is just as a measure of a quantity. Its just the amount of energy. If you imagine energy as an object. One of those objects is a joule. In your example 1400KJ to heat up 100g of water. You need 1400000 joules to heat that water. Obviously there is a more complicated definition but for GCSE just understanding it is a measure of energy is usually enough. Hope it helps? =/