ATP are the molecules that are necessary for most of the biological reactions to happen. It is like all of the reactions have a cost and ATP molecules are the currency that pays that cost. "Saving money", in the ATP context, could be eating and storing the energy that comes from carbohydrates and lipids into body fat which is our energy bank. Later, when needed, this fat would then be converted into glucose, which is the main fuel for ATP production through glucolisis.
