What is it? And, how does it affect performance? thanx.

 

Do you mean port and polish? This usually involves enlarging and smoothing out intake and exhaust passages. I don't think anyone actually "polishes" heads anymore, though. Shaping the intake and exhaust ports optimally improves flow in and out of the combustion chamber, which equals more power. Bigger isn't necessarilly better, so an experienced tuner is key.

 

The polish usually refers to the combustion chambers.

It affects performance by allowing your engine to flow more air.

More air = more power.