Pastry flour is a relatively low-protein flour that is often called for in making biscuits, cookies, pie crusts, and pastries. The protein content of any given type of flour determines how tender, strong, elastic, stretchy, pliable, etc., the dough is that you make with it, and also the texture of the finished bread, waffle, cookie, croissant, etc.
Bread flour, for instance, weighs in between 12% an 13% protein, and helps produce wonderfully well-risen, chewy loaves of bread. Cake flour, at the low end of the spectrum, 5% to 8% protein, is much less elastic, and helps produce wonderfully tender cakes. Pastry flour is up only one notch, at 8% to 9% protein, and lets you create baked goods with a little more body and texture than cake flour, but still with the tenderness one associates with a well-made biscuit or pastry.
It can be a challenge to find pastry flour. Even well-stocked supermarkets seldom carry more varieties than cake flour, all-purpose flour (9% to 12% protein), and bread flour. If you can’t find pastry flour, you can mix you own by combining cake flour and all-purpose flour in a ratio somewhere between two parts cake flour to one part all-purpose and one part cake flour to one part all-purpose.