Everything is made up of atoms. Atoms are made up of protons, electrons, and neutrons. The nucleus of the atom contains neutrons which have a neutral charge and protons which have a positive charge. Electrons orbit the nucleus and have a negative charge.
If you have the same amount of protons as electrons, you’d have a neutral charge. If the atom loses an electron, you’d have a positive charge. If the atom has more electrons, the atom would be negatively charged. Electrons can jump from one negatively charged object to a positively charged one and will balance the charge. The energy used when the electrons move is the voltage.