Crispbread, known as knäckebröd in Sweden, is a dry, hard, thin flatbread made from rye (or other grain) flour, water, and salt, baked until crisp. That dryness gives it a long shelf life without refrigeration.
It's traditional in Sweden, Norway, and Finland, where some sources trace it back as far as the 6th century AD as a way to store bread through the winter, traditionally baked in flat rounds with a center hole for hanging on poles.
It's served with butter, cheese, or cured fish, crumbled into soups, and used as a gluten-conscious substitute for crackers or breadcrumbs.