what is phase changing and what does it do- thank you

Depends on what you are trying to find out. Phase change can be freezing water to become solid ice, or boiling water to become steam. Alernate current to Director current is also considered as phase change. Setting a log on fire so it turns into ashes could be phase change also.

When the tape on the bottom of your heatsink changes from a solid to a nearly liquid putty, that a phase change. When you turn your rig off and it changes back into a solid again, thats another phase change. I believe it happens at about 58~60°C Skip

When this phase change occurs does it have an effect on the performance of the HSF?

The effect it has is the desired one of softening the thermal pad to the point that it flows into all the microscopic imperfections in the heatsink and cpu. The heatsink is under pressure from the retention clip insuring a good interface between the two. Let's say you just installed your heatsink. You fire up the machine and the cpu heats up pretty good because of the poor interface between it and the heatsink, the thermal pads changes from a solid to nearly liquid and gives the desired interface that allows maximum heat transfer from the cpu to the heatsink. Next time you start your rig, it doesn't heat up enough to melt the thermal pad again. That's the desired reaction and a good reason to replace the thermal pad with another or thermal paste if you ever dismount the heatsink. Skip

This tells about phase changeing http://www.extremetech.com/article2/0,3973,1124222,00.asp

So a thermal pad's effectiveness only increases when the cpu heats up unlike a thermal paste which constantly provides a good interface between the cpu and HSF because of its "semi-liquid" form?