When something is spectral, it has a ghostlike quality; it seems to vanish or disappear. You may have heard of ghosts referred to as "specters": that means that they're spectral.
You may recognize the spect part of the adjective spectral from words like "spectacle," "spectrum," and "spectacular." All of these words share a similar meaning, which comes from the Latin verb specere, "look at, view." When something is spectral, think of it as something you can see but not touch — though it won't always mean that literally. The young model was so skinny she had a spectral quality: she almost seemed to disappear.