It is a specific flour that gives bread a lighter texture. It is a special blend of flours. King arthur sells it in a blue & white package, gold medal is a almost all yellow bag.
There are also differences in whole wheat flours as far as bread flour and pastry flour. You won't likely find these different flours in the regular grocery stores, but you might in a store like Whole Foods.
Or if you grind your own grains one day. No guffawing, we grinders all snorted at the idea at one point, too! It's an addiction, and we don't want therapy!