What does the term rolled off highs mean ?

well yes in a way it is because 'perfect' sound on a chart would be a flat line but some may prefer a bit of a roll off so the tweeter is not too bright in a reflective room (for instance).

stereos are far from perfect so a little roll off can be good or can be viewed as bad

there is no absolute

Generally, when someone describes the highs as "rolled off", they feel like the high's are dropping off before 20,000 cycles. Generally, a flat frequency response is desired between 20Hz-20kHz (typical child hearing range). I would guess that many/most of the users here can't really hear much above 15-16 kHz.

Philjolet is correct in calling it arbitrary. Some prefer a perfectly flat frequency response, others prefer rolled of highs to keep your average recording from getting too bright.

My tweeters are suppose to be good to 60KHz; WAY beyond my hearing range or anyone else's. Why? Because it is felt by many that what happens outside the audible range affects sounds within it. "Rolled Off" is a pejorative implying that there is insufficient treble to accurately reproduce the music.

Less high information. Like if you had a treble tone knob and rolled it backward.