A battery may look like a small box that stores energy. But a battery does not just store electricity. A chemical reaction happens inside it.
When you use a battery, this reaction helps electricity flow. The electricity can light a lamp, move a remote control, or power a phone.
However, the materials inside the battery are used little by little. As the materials run low, the battery becomes weaker. Finally, it cannot give enough power to the device.
In a rechargeable battery, electricity can push the reaction back in the other direction. Even so, many batteries slowly become weaker after many uses.