Atoms are the building blocks of the universe. Depending on the mood they're in, some atoms like to be with other atoms. A bunch of atoms together, in a specific shape, is called a molecule. Well, chemists, biologists and physicists can mix and match molecules and atoms in high-tech soups.
Special combos can give off energy.
The energy itself is contained in the bonds between the atoms and molecules. Many batteries run on chemical energy. The batteries we buy in stores are useful little packets of energy to power things like radios. But these batteries are weak and run out of juice eventually. Well, at least by rubbing the oxygen in air together with hydrogen.