Porting are the holes that are drilled in that front portion of a paintball barrel. Simply put porting does two things makes the paintball gun quieter and aids in accuracy.
By adding porting to a barrel, air escapes the barrel from the sides, reducing the amount of air that exits the barrel at the tip. Removing the access air from the tip of the barrel aids in reducing the loud popping sound.
Porting also helps with accuracy a bit. The porting helps draw away extra air away from the ball and help reduce turbulence when the ball exits the barrel.
While porting is a good thing too much can also be detrimental. There is a sweet spot with porting, if you add too much there can be too much air loss and a drop in velocity. A drop is velocity can be overcome pretty easily but it usually comes as a result of increasing gun pressure, thus making the gun kick more, less air efficient and possibly break more paintballs in the breech. Most manufacturers have figured out this sweet spot, if you do see a barrel with more porting than others be skeptical.
It should also be noted that the pattern does not affect the flight of the ball noticeably.