What it means to me is that the treble sound isn't thin, harsh and/or metallic but neither is it shut-in or too rolled-off as the frequency rises. Essentially it is important for higher string sounds in orchestral music, where you want to get a strong impression of the real timbre of the violin - enough detail to really enjoy the full extent of the playing but no strangulation of the tonal body of the instrument. Of course, it is also important for other high register instruments and voices as well, but upper strings are a real tell-tale of how good the treble performance of a system is.