A rusk is a biscuit such as Zwiebach made from sliced yeast bread, then baked until it is brown and dry. Rusk flour is pulverized rusk. Rusk flour is also a term used to described browned bread crumbs and it is savory rather than sweet.
The flour does not have a long shelf life so it is best to purchase your own. You can buy rusk biscuits in most grocery stores. Simply place the rusk in a food processor and grind to the desired consistency. See other substitutions below.