Definition
ATP is the molecule that holds the energy living things can use right away to do work inside the body. It takes energy from food, stores it, and hands it out in small bits right where it's needed. Once it's spent, it gets recharged, again and again, nonstop.
Your body has tiny rechargeable batteries
Think about a wireless game controller. You can't keep it plugged into the wall outlet while you play. So electricity gets stored in a small battery inside. You play on that battery, and when it runs low, you recharge it.
Your body faces the same problem. Rice and bread are packed with energy, but you can't shove a spoonful of rice straight into a leg muscle. The energy in food comes in chunks that are too big and too rough for a cell to use directly.
So cells break that energy into small pieces and load it into tiny batteries. Those batteries are ATP. It's energy converted into a form a cell can plug into and use right away.
Power comes out when one tail piece drops off
This battery has three phosphates attached in a row, like a tail. A phosphate is a small piece built around the element phosphorus. When the phosphate at the very end breaks off, the cell gets power it can use.
That power makes muscles contract so your legs can move. It lets nerve cells pass signals along. Even your body warmth comes partly from this: heat is released as the battery is charged and used. After giving up one piece of its tail, the battery has only two phosphates left. It's drained.
A drained battery doesn't get thrown away. The mitochondria, the power plants inside your cells, use pieces of food and the oxygen you breathe in to reattach a phosphate, and they make the largest share of ATP this way. In moments when oxygen can't reach the cell fast enough, cells also use another route that recharges in a hurry without oxygen. Either way, the battery is charged back to full.
So your body uses and recharges the same batteries over and over. Rather than making new batteries and stockpiling them, it keeps recycling the ones it has.
A little more precisely
This battery isn't a warehouse for storing energy long term. The ATP in your body is only enough for a short burst. But because it's recharged the moment it's used, you never run short during the day.
Energy for the long haul is kept somewhere else. Fat, and sugar packed away in the liver and muscles, act as the storage rooms. Your body pulls materials from those stores to recharge the batteries, like runners passing a baton. So a big storage room doesn't mean your body is holding lots of batteries at once.
The idea that energy is trapped in the bonds between phosphates is also just a picture to help us imagine it. It's closer to the truth that the state after the piece breaks off is more comfortable, and the power comes from that difference. From bacteria to trees to people, every living thing made of cells uses this same battery.
Common misconceptions
ATP is the body's warehouse for storing energy over the long term.
Your body holds only enough ATP for a short burst. Long-term energy is stored as fat or as sugar packed into the liver and muscles, and it's converted into ATP when needed.
Only complex living things like humans and animals use ATP.
Bacteria and plants use the same battery. Plants first load the energy they get from sunlight into ATP, then use that power to make nutrients.
Where you meet it
When you sprint up a flight of stairs, your leg muscles burn through ATP quickly to produce force.
Even while you sit still reading a book, your brain cells use ATP to send and receive signals.
In one sentence
ATP is a tiny rechargeable battery that cells plug into directly. Cells pop off one phosphate to get power, then reattach it to recharge, again and again.