Historically, very significant for sailors, soldiers, pioneers and explorers. You can be sure that hard tack found its way into the pockets of many, from John Franklin to Ernest Shackleton and countless more besides.
The difference between life and death for many.
Flour, salt and water are the ingredients of traditional hard tack. Essentially, unleavened bread that is dry as dust.
Its low water activity makes it very unappealing to microbes; it can survive a very, very, very long time in the hull of a ship, or in your backpack, or in your cupboard.