If the spectrum of a quasar (or, more generally, an AGN or QSO) contains an absorption feature that was not produced by the quasar itself, that means that the quasar’s light has shined through some material or object that absorbed some of that light. This kind of “quasar absorption line” can be studied by the effect it has on the quasar light, even if the absorbing object cannot be directly viewed by its own light emissions.