Port and Polish...

[Yossarian]

What does a Port and Polish do exactly?

[vvtlikick]

When DONE CORRECTLY, porting and polishing opens up and smooths the air passages in the engine (manifolds and head) allowing more air into/out of the combustion chamber.

I'm no engineer, but I think basically you want to keep the passages small (for a high-velocity flow) and free-flowing (for a large flow volume). These are somewhat contradictory goals, so quality work takes experience, R&D, and trial-and-error. :)