Smooth tubes are the intake tubes from the filter box to the intake manifold. The difference between stock and smooth is that the stock tubes are corrugated, resulting in the air being "disturbed" as it passes through and thus lower horsepower. The smooth tubes combined with K&N or similar high-flow filters will provide an amazing 7-15 horsepower boost - others have seen even higher. Note that the ACR, which is rated at 460HP vs. 450HP for the RT/10-GTS, has only these two modifications (tubes & filters) to get that factory horsepower rating. Of course there are loads of other goodies that come with an ACR package to justify the $10K+ price difference, but as far as engine mods go, that's it. Hope this helps!
Chris