If someone knows, can you tell me what is the difference beetween spectral lines and resonance lines?

I know that ionised substances emit most of their emitted electromagnetic radiation at frequency corresponding to their resonance line, but what is the reason?

And if I take an example, sodium, which mainly emits at two lines, called D1 and D2, are they both resonance lines or only one of them? And if they are both resonance lines, does sodium have other resonance lines also?

"Spectral line" means one line from the measured spectrum, regardless how that was created. Its width, so wavelength/frequency/energy range it covers corresponds to one unit of resolution of the measuring equipment you are using. So the term is more related to the way how the spectrum is observed.

"resonance line" means a part of spectra formed by some kind of resonance (or apossible energy state transition, when looking at the thig via Faynmon's duality laws; the possible electron energy transitions when capturing/releasing photons manifest itself as an optical resonance, when described using wave theory equations). So the term is used more in a context of how these photons are physically generated (or absorbed, e.g. in the case of selfabsorbtion inside HPS).

So the Sodium "D1" and "D2" lines are most often a single spectral line because most spectrum analyzers can not distinguish them, but two resonance lines, as each is related to a different energy transitions, even when very close in their values (so they both yield practically the same light).

Thanks for answer!