Can somebody tell me what yellow tipping means?

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&lr=&q=%22yellow+tipping%22gas+flame&btnG=Search

Where are you seeing the term?. or in what conversation did it come up?

Yellow tipping really is not a huge concern. The primary air adjustment on the burners could be misadjusted. Imputes in the gas. Ash and dust falling into flame from heat exchanger. Or possible disruption in the flame from a cracked heat exchanger, but by no means does this mean you have a crack.

All flames yellow tip to a certain extent. What you don't want is a "Lazy" flame that wavers or is mainly yellow. You are looking for a strong, tight blue flame until the very end...