What does the term rolled off highs mean ?

turning the treble down is the same as rolled off highs.

If you saw it on a chart the end of the line would literaly roll off (slope down) at the end.

is there a certain frequency that should be achieved before rolling off ?

No, it is a arbitrary thing...

whatever device it is that is 'rolled off' determines it.

The way i have interpeted the reviewers comments about rolled off highs made me think it was a derogatory term ?

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