What is "00 flour"?

"00" or "Tipo 00" flour is wheat flour, from, usually, Italy. It is designed for pizza dough and some pastas. What makes it unique is that it is ground finer than AP flour and contains low levels of protein (lower than even Cake flour), meaning, the resulting dough can be handled more and develop less gluten. Discerning pizziaolos know this and this is why the kneading process is considered, the most crucial step in making perfect pizza. King Arthur does make a suitable "Italian-Style" flour, but many consider the Caputo variety to be end-all when it comes to "00" flour.

I've found the Wondra is a reasonably good substitute, and is commonly available in grocery stores.

King Arthur Flour has it for sale as Italian finely ground wheat flour. I use it for pizza dough, focaccio , etc. Check out the KAF website for lots more detail