What do lines in an elements line spectrum represent?

Electron transfers between energy levels.

Explanation

An atom contains an infinite number of energy levels. An electron can exist within any one of them, but not in between. Electron can move from one level to another. However, those levels differ in energy. Energy has to conserve.

An electron loses energy when it moves to a level of lower energy. The energy is released as radiation. The frequency of the radiation depends on the energy difference. Those radiations combine to produce an emission spectrum. An electron gains energy when it moves to a level of higher energy. This transfer happens only if there's an external source of energy. For example, directing a beam of white light on the atom. The beam of light would excite the electron. Again, the frequency of the radiation depends on the energy difference. Only frequencies that corresponds to transfer will be absorbed. Those radiation will eventually be released in all directions. As a result, strength of some of the radiations will be weakened. A continuous spectrum with those frequencies weakened is an absorption spectrum.