my contractor has suggested that we pop the grain before we apply our stain on red oak hardwood floors. My stain in a walnut color with a satin finish. I want them to look nice, but I don't want them to be way to dark. Everything I read said pop the grain, but I am not sure. Perhaps someone can tell me the advantage as well as disadvantage.

K, well I dont want to sound like a "smart-azz" but if your contractor recommends popping the grain, and everything you have read says to pop the grain...........
Anyway, what this does is opens up the pore of the wood to allow for deeper penetration and hue of the stain.
It is a standard process for deeper colours like walnut, and also allows for a more consistant look to the stain itself. It evens out the sanding so that there are no hard spots from buffing over a high point in the floor, etc.

Sorry for being a wiseguy, not having the finest of days, but hope I clarified things for you.

You could ask your contractor to make sample boards for you. One showing how it looks with the grain being popped and one without. It will help you decide which technique will give you the desired color.