As someone who prefers a “warmer” sound and feel my Wharfedale Denton 80th speakers fit the description well, to me it means that the overall sound is smoother, softer, and has more emphasis on the mids and low end whereas the highs will be rolled off to some varying degree and less in your face or harsh. Veiled is a term I’ve seen people use and I think to some degree that’s true, but not in a way that the sound is muffled at all, just that it’s more controlled to prevent the highs from peaking and getting harsh.

I’m definitely a novice when it comes to being able to describe these things and haven’t heard many “hifi” speakers, but from the couple years I’ve been in this hobby, this is how I’ve come to define warm compared to bright.