Roughly, Aufbau Principle says that in an atom or an ion, electrons enter the shell with low energy first before entering the shell with high energy.
An atom has shells around its nucleus. Electrons enter these shells and orbit around the nucleus of an atom. To say about the properties of these shells, the shell nearest to the nucleus has the lowest energy. The farther the shell from the nucleus, the higher its energy. Aufbau Principle states that electrons enter the closest shell to the nucleus first since it has the lowest energy. When that shell has its maximum number of electrons is can hold, then the next electron will enter the second-closest shell to the nucleus, which has higher energy than the first shell.
