Parus, porting is altering the shape and size of the inlets and outlets to make the engine breathe more effectively.

You are aiming to increase the velocity and amount of air into the cylinder (allowing more fuel and a bigger bang), then trying to get it all out again as quickly as possible.

Here is a reasonable link to something relating to cars.

Hope this helps
http://www.torquecars.com/tuning/Porting-polishing.php" onclick="window.open(this.href);return false;