Atom has discrete energy states (energy level). When atom transits from a high energy level to a low energy level, it radiates line emission with some specified wavelength, corresponding to the energy difference between both levels.
In particular, line emission after the recombination of, e.g., ionized hydrogens with electrons is called recombination line.
Spectral line of hydrogen
Energy level of hydrogen
Model of iron atom
Line spectra from an accretion disk

Vibrational transition of H₂O

Rotational transition of H₂O

Molecules, which consist of multiple atoms, can vibrate or rotate around an axis. When a vibrational or rotational state of molecules discretely changes, obeying quantum dynamics, they emit or absorb line radiation. Electron energy transition as well as vibrational and rotational transitions make moleclar lines.

Cabon monoxide CO, which is a typical two-atom molecule, consists of cabon atom (●) and oxigen atom (●). This carbon monoxide can vibrate in the direction of a combined axis, while can rotate around the axis.

In order to express such vibrational or rotational transition, we use
CO (v = 2-1)　from level 2 to level 1 of CO vibrational transition
CO (J = 2-1)　from level 2 to level 1 of CO rotational transition

Vibrational transition
Rotational transition

A three-dimensional molecule like ammonia emits electromagnetic radiation, by reversing its configuration.
Ammonia molecule NH₃ consists of nitrogen atom (●) and hydrogen atom (●) in the form of a tetrahedron.