On board ships, what was 'hard tack'?

Hardtack (or hard tack) is a simple type of cracker or biscuit, made from flour, water, and sometimes salt. Inexpensive and long-lasting, it was and is used for sustenance in the absence of perishable foods, commonly during long sea voyages and military campaigns. The name derives from the British sailor slang for food, "tack".

Because it is so hard and dry, hardtack (when properly stored and transported) will survive rough handling and temperature extremes. To soften, it was often dunked in brine, coffee, or some other liquid or cooked into a skillet meal. Baked hard, it would stay intact for years if it was kept dry. For long voyages, hardtack was baked four times, rather than the more common two, and prepared six months before sailing.