It's not an "official" hvac term, but if you're speaking of the flame on a burner, it probably means too much yellow; that is, past the "yellow tip limit". This can be caused by a variety of problems...most of which can be fixed by adjusting the flame.
If a reputable service tech says you're "yellow tipping", then the solution is to have the flame adjusted.
Here's some reference for it:
http://www.google.com/search?hl=en&l...me&btnG=Search
Where are you seeing the term? ...or in what conversation did it come up?