molecular energy levels
A molecule can store energy only on a set of fixed rungs, like a ladder, never on the smooth spaces between them. It can hold energy in its electrons (which orbital they occupy), in the vibration of its bonds, and in its overall tumbling rotation — and each kind comes in its own ladder of allowed amounts. Molecular energy levels are this whole set of permitted energies the molecule is allowed to have.
Because energy is quantised at the molecular scale, a molecule cannot take just any energy: it occupies one of a discrete set of levels. These levels stack at very different scales — electronic levels are far apart, vibrational levels are closer together, and rotational levels are closer still — so a full picture looks like coarse rungs (electronic) each subdivided into finer rungs (vibrational), each subdivided again into the finest (rotational). A molecule absorbs or emits light only when it jumps between two of these allowed levels.
This ladder is the foundation of all spectroscopy: the spacing between two levels sets the frequency of the light involved, which is why electronic jumps need visible or ultraviolet light, vibrational jumps need infrared, and rotational jumps need microwaves. The honest qualification is that the neat separation into electronic, vibrational and rotational is an approximation — the three kinds of motion are not perfectly independent — but it is an excellent one and underlies how the whole subject is organised.
Think of a molecule's energy like a building: floors set far apart are the electronic levels, the steps within each staircase between floors are the vibrational levels, and the individual stairs are the rotational levels. Microwaves move it one stair, infrared one flight, and ultraviolet a whole floor up — three sizes of jump, three regions of light.
Coarse electronic rungs, finer vibrational ones, finest rotational ones.
The three kinds of level differ enormously in spacing, which is why each spectroscopy lives in its own region of the electromagnetic spectrum. This separation is what lets us study rotation, vibration and electronic structure almost independently.