A sort of elemental fingerprint
The further a shell is from the nucleus, the higher the amount of energy is that it contains. As atoms strive towards a minimally energetic state, the electrons would prefer to drop down to a lower shell; however, since the shells have a maximum capacity, the electrons cannot do so unless there is a “free” spot for them to take.
This preference of electrons to “drop down” is made use of in emission spectroscopy: Energy is applied to the specimen (directly by applying a voltage, or with an electron beam), and electrons are “knocked out” of the inner shells. Outer electrons can now drop down and take these free spaces; however, since outer shells possess more energy, the electrons must get first rid of the surplus energy from the outer shell. This emitted energy takes the form of radiation, and is equal to the energy difference between the inner and outer shell. As the amount of energy in the shells is characteristic for each chemical element, these “packets” of emitted energy are also characteristic for each element – a sort of elemental fingerprint.