The electrons in an atom are arranged at different distances from the nucleus. Each distance is an energy level. The further out the level, the more energy the electron has.
If an electron absorbs electromagnetic radiation, it gains energy and moves to a higher energy level. We say it is excited. This can happen to inner electrons as well as outer ones. Later the electron falls back to a lower level and emits the energy it loses as electromagnetic radiation. So the arrangement of electrons changes when radiation is absorbed or emitted.
If an outer electron gains enough energy, it can leave the atom completely. The atom then has more protons than electrons, so it has an overall positive charge and is a positive ion. Losing outer electrons in this way is called ionisation.
