The Aufbau principle, also known as the building-up rule, is a method for determining the configuration of electrons in an atom or ion. The rule states that the electrons surrounding a nucleus can be "built up" by first occupying the most stable orbitals, often those of the lowest energy level.
This principle can be illustrated by examining the distribution of electrons in an atom of carbon. The six electrons surrounding a stable carbon nucleus would be built up the following way - first, an electron would be added to the 1s orbital. This is the lowest energy orbital and closest to the nucleus. By applying the Hund rule in conjunction with the Aufbau principle, one can determine that the second electron will also occupy the 1s orbital. The Pauli exclusion principle then yields that any further electrons must be added to the 2s shell. After both the 1s and 2s orbitals are full (for a total of four electrons), electrons may be added to the next highest energy level, the 2p orbital; the two remaining electrons can be found here. This process yields the electron configuration of carbon as 1s2 2s2 2p2.