The atom consists of protons, neutrons and electrons. The protons and neutrons reside within the nucleus whereas electrons revolve around the nucleus in fixed orbits or shells. These shells are complex regions further consisting of subshells and orbitals.
Electrons are arranged in shells (energy levels) around the nucleus.
Energy levels are represented by the symbol n and are numbered according to how far they are from the nucleus.
The lowest energy level n=1 is closest to the nucleus, the energy level n=2 is further out and so on.
The different energy levels are designated by the alphabets K, L, M and so on.