Bread flour has a 12 or 13% protein content, making it stronger than regular all-purpose flour. If you don't have this ingredient, you have other options. Either use all-purpose flour or bear in mind the pound cake won't have the exact same texture or replace 1½ teaspoons of all-purpose flour with 1½ teaspoons of vital wheat gluten. To be honest, it really doesn't make that much difference. I do like to use bread flour in bread recipes though because it gives such a nice 'chew'.