According to Bohr's atomic theory, electrons in an atom revolve around the nucleus in their fixed path called orbits.
These orbits are also called energy levels and each orbit have a specific energy associated with it.
Because while revolving on this particular orbit electron have some specific amount of energy which allowed electron to remains in these orbits and so these orbits are called energy levels.
These orbits are named as K orbit, L orbit, M orbit, N orbit and so on, where K is the first and innermost orbit having lowest energy. Energy of orbits increases in the order K < L < M < N and so on.