Bread flour is much like all-purpose flour, save for the fact that it’s often made from just a single type of hard wheat (versus a blend). It’s high in gluten, as you’d expect (usually around 13 percent) and because of that it produces the highest, fluffiest and chewiest breads. Oftentimes millers will add special “dough conditioners” to a bread flour to help maximize its loaf building potential: ascorbic acid or (increasingly rarely) potassium bromate, which helps oxidize the bonds on the ends of gluten molecules, helping them to form even longer, stretchier chains. Sometimes a little ground, malted barley (malt powder) is added. The malt introduces enzymes into the mix that speed the breakdown of starch into sugars, which feeds the yeast.
If you don’t have access to bread flour at your local market, you can use a national brand all-purpose, which is going to have quite a bit of gluten anyway. “Spiking” it with a little vital wheat gluten (by adding about 1 teaspoon per 5-ounce cup of all-purpose flour) transforms it into a near exact replica.