selection rules
On a board game, not every square is reachable in one turn — the rules say you may move one space, or jump like a knight, but you simply cannot make certain moves. A molecule faces the same restriction when it tries to change energy level by absorbing or emitting light. Selection rules are the list of which jumps are allowed and which are forbidden, telling you in advance which lines will actually appear in a spectrum.
Selection rules are the conditions, derived from quantum mechanics and the symmetry of the molecule, that decide whether a given transition between two energy levels can occur by absorbing or emitting a photon. An allowed transition is strong and shows up clearly; a forbidden one has essentially zero probability and produces no visible line, even though both energy levels exist. The rules usually take the form of simple statements — for example, a vibration must change the dipole moment to absorb infrared, or a rotational state may only change by one step.
They matter because they explain a spectrum's actual appearance: why some expected lines are missing, and why each technique is blind to certain motions. The honest nuance is that 'forbidden' rarely means impossible — it means very weak. Faint forbidden lines do appear when the idealised assumptions break down slightly, which is itself a useful clue about a molecule's finer structure.
Oxygen and nitrogen make up most of the air, yet neither absorbs infrared, so they do not warm the planet the way carbon dioxide does. The reason is a selection rule: their perfectly symmetric stretch causes no change in dipole moment, so that vibration is infrared-forbidden and stays invisible to the heat-trapping mechanism.
Which transitions are allowed decides which lines a spectrum can show.
Different techniques have different selection rules, which is why they complement one another: an infrared-forbidden vibration may be perfectly Raman-allowed. A line missing from one spectrum is often present and informative in another.