Please explain Aufbau's principle in detail. I do not understand it at all! :( and also with example please.

According to Aufbaus principle, in the ground state of the atoms, the orbitals are filled in the order of their increasing energies. In other words, electrons first occupy the lowest energy orbital available to them and enter into higher energy orbital only when the lower energy orbitals are filled.The order in which the energies of orbitals increase and hence the order in which the orbitals are filled is as follows:1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.